King's Quest IX is a hypothetical and unreleased King's Quest sequel set after King's Quest VIII: Mask of Eternity. If the ninth sequel would have been published, it would not have had the numeral in the title (like King's Quest (8): Mask of Eternity) due to marketing decree that Sierra would no longer include installment numbers in their games to follow the trends of the day. 
There have been a number of failed attempts making a ninth KQ or at least resurrecting the series for new audiences.
There have been a number of attempts at making a ninth King's Quest game and/or rebooting the series. The only developer to successfully produce a game was The Odd Gentlemen's reboot (though this is not the fabled King's Quest IX).
Notably like KQ8, the next game would not have a numeral in the title due to reasoning of marketing department: “9 was not in the title, because Marketing decreed that several other big companies' series were no longer including installment numbers, so we should follow the naming trend.”
This article discusses ideas and suggestions made by Roberta Williams, if she had been allowed to work on KQ9, after KQ8. These mostly hinged on the success of KQ8, and if Sierra's owners had chosen to ok development on a new game with her at its helm (1998-1999). Duringf this period Mark Seibert and Roberta discussed a few ideas for the next game while they were discussing the "new" release of then current King's Quest 8.
While KQ8 did see enough success that Sierra green light a follow-up, Roberta was not brought in. The follow-up was later cancelled as Sierra Studios Seattle, laid off employees and collapsed.
The article also goes into the known history of other attempts to make ninth KQ, and finally the on-going published attempt to reboot the series with the new series King's Quest: Adventures of Graham.
The first was an attempt was by Sierra then owned by Vivendi Games, following the reasonable success of King's Quest VIII, headed by Mark Seibert (who was one of the heads on KQ8), but without Roberta.
- King's Quest: Twins of Change (1999-2002), a game designed for consoles by Mark Seibert for Vivendi Games (under the Sierra label at Sierra's Seattle studios).
- King's Quest (Silicon Knights) (2007), a game in development by Silicon Knights, to be produced by Vivendi Studios.
- King's Quest (Telltale Games) (2011), a game in development by Telltale Games and published by Activision.
- King's Quest: Adventures of Graham (aka King's Quest) (2015), a game developed by The Odd Gentlemen, and produced by Activision under a resurrected Sierra label) Note: This is not considered a true KQ9 according to the developers themselves.
From Adventure of Graham's perspective in the alternate reboot universe, there is a hypothetical KQ9 somewhere between the end of King's Quest VIII: Mask of Eternity and the episodic King's Quest: Adventures of Graham. It would take place when Graham is roughly in his 70s see KQ9 timeline (in the rebooted timeline). In the new timeline, it may roughly coincide with King's Quest Chapter V the final episode of the episodic series. However, it must be noted, that events of the reboot universe occurred differently than they did in the original universe (with events of each game having occurred much differently than original games).
Despite the new series King's Quest: Adventures of Graham episodic series being in an alterate universe, it is the ninth published original King's Quest adventure title (but not considered KQ9).
The official hint book for KQ8, makes reference to King's Quest IX being the next game to be released in the future, assuming that King's Quest 8 sold well, and that Roberta Williams was still going to be at its helm.
It would not have been called King's Quest IX since before the release of KQ8, marketing made it clear that developers were not to use numerals in their titles, because other big-name companies had done away with the practice at the time. Hence, even roberta refers to this one as "King's Quest (9)".
Roberta Williams had her own conceptual ideas for King's Quest IX (or future King's Quest games) she mentioned around the release of KQ8 between 1998 and 1999. Though these never went into production. However, for the most part She brushed away questions about any future sequels in the series. However she did discuss an idea or two about what a future game might hold if she could have produced another game in the series...
Unfortunately, she was never able get Sierra greenlight development on a new game. Before she left Sierra (shortly before the release of KQ8), she forced to sign a non-competition agreement which prevented her from making computer games for the next five years, or anything that might compete with Sierra's products. It took some further negotation, and a small list of changes before she even allowed her name to be put on the box. She only came back to market the game, after its release. It was during this time, she discussed some potential ideas for KQ9 or later games in interviews. But at the time she had no idea if CUC would have contracted her to make the new game or not (and she knew it would only happen if the game had been a success, which she believed it was). However, CUC was not one for contracting or assigning work on classic series to previous employees (see Al Lowe with Leisure Suit Larry who wasn't assigned to work on Magna Cum Laude, and Jane Jensen who was given a pink slip after release of Gabriel Knight 3), so Roberta was never contacted again.
Thanks to Ken Williams and Roberta Williams we can have somewhat of an idea what King's Quest 9 (and King's Quest 10) might have been about in this summary design doc given (non-exclusively) to the King's Quest Wiki with both Roberta and Ken William's kind permission to publish: The story would have involved the villain Rasputris, conquering the kingdom from within, as a hypnotic advisor to the King and Queen. Rosella had uncessfilly attempted out him, and then brought help from her brother Alexande, and Connor, now the Marquis of Daventry (the rest of the design doc can be read on the King's Quest IX (Roberta Williams) page).
King's Quest: Twins of Change
Following the success of King's Quest 8 (it was a "minor hit" as Ken Williams calls it, and maybe not what everyone was hoping for), and a year or two after Roberta left the company. Sierra started development on a ninth King's Quest game in 1999 under Sierra branding but to be published by Vivendi Games. It had nothing to do with any of Roberta William's earlier ideas, and conceived to be an original take on the series. However due to collapse of Sierra, it ended up being cancelled by 2002.
The new King's Quest was one of the three games from prominent adventure franchises in Sierra On-Line's portfolio (King's Quest, Space Quest, and Leisure Suit Larry) that would receive a new version adapted to the gameplay styles that interested the common gamer at the time. It was to be a reinvention of the series. Like Escape Factory's Space Quest update, King's Quest would have been a cartoon-style action title with very little semblance to the previous titles in the series (even compared to KQ8). Unlike SQ and LSL, KQ never made it past prototype.
It was headed by Mark Seibert, and the writer was Cindy Vanous.
The main characters were Alexander and Rosella, though they'd been hit by wild magic and weren't exactly human any more. We were just calling them Alex and Rose in the script, since that's most likely what they'd call each other familiarly.They each had a series of animal forms they could transform into, though you'd have to unlock them over time, to solve various puzzles. Rose ended up tiny, so her forms were small flying creatures, and Alex had larger, stronger forms.
The game had roughly started in 1999 (and had nothing to do with Roberta William's ideas for the game). The project was given over to Mark Seibert who was the director on King's Quest VIII: Mask of Eternity. So it would have involved at least some of the original developers. It was going to be designed as a more console style-action adventure game like Legend of Zelda, based on known information. This one basically died when Sierra Seattle studios finally died under Vivendi Games.
The work on the game probably began around 1999-2001. But may have been 'wounded' as early as Chainsaw Monday, February 22nd, 1999, and been affected by the domino repercussions of that event (taking out Sierra's Seattle studios as well, a few years later).
Passed onto Silicon Knights
With Sierra gone, and Vivendi Studios began to phase out the label, but continued to look for more developers to resurrect the Sierra IPs in new ways. Silicon Knights was in talks with Vivendi, and was negotiating development plans for King's Quest (c. 2007). Silicon Knights began development to a King's Quest sequel apparently using Epic's Unreal Engine (Unreal Engine 3) It is said that Silicon Knights got as far as a prototype before it was cancelled. The exact timing of when this game began and ended is not clear, but apparently still in development around the time Vivendi was sold to Activision, and up to the point when they shut down a popular fan game based on the King's Quest series (starting a letter campaign to save it). Knowledge of this game was discovered in a 2011 lawsuit between Silicon Knights and Epic games over the use of the Unreal engine. During the lawsuit with Epic, Lloyd's methodology for determining the potential value of earlier and cancelled projects was similarly unconvincing to the court.     Ultimately the judge ordered that Silicon Knights destroy any code related to any games that had used code from the Unreal 3 engine, so if there was anything related to the prototype its probably been destroyed, and there are no existing photos from this game.
Telltale Gains the Developmental License
Activision continued to look into viability and possibility of using and resurrecting some of the old Sierra IPs, it gained when it bought out Vivendi Studios. Activision's first attempt at this was passing the development rights of Sierra IPs over to Telltale Games. Telltale decided to start with King's Quest as the first game they would resurrect, with the possibility of resurrecting other IPs if it had been successful.
King's Quest was to be an episodic series of games in development by Telltale Games. Thought to be a 'reboot' but may have been intended to fit into the continuity of the original series (though possibly in the 'way' that The Odd Gentlement material 'fit' but was still an rebooted universe with an alternate history). Telltale sat on the IP apparently doing very little with it, while they were working on other successful series such as The Walking Dead. Activision chose to take back the licenses and claimed they would look at the possibility of doing a new series in-house. Though there were at least several other interested parties including Replay Games (Leisure Suit Larry Reloaded) that were turned down. Activision would ultimately pass development rights over to The Odd Gentlemen, but keep publishing rights in house. Since Telltale only sat on the developmental licenses of Sierra series, its not clear if any concept or ideas of King's Quest were even discussed in any detail. There are no screen shots or artwork related to this part of King's Quest history, other than Telltale's use of a golden variation of the King's Quest title (similar to the one used in King's Quest VII box), and a small blurb:
- Return to the Kingdom of Daventry as Telltale is working on new adventures based on the classic adventure game series!
Rebooting the King's Quest Series
King's Quest Reboot: Adventures of Graham
Although King's Quest: Adventures of Graham is the ninth game developed in the series (or tenth if you count King's Questions or the King's Quest I SCI remake), its not technically King's Quest IX. It is instead a reboot of the series, seen as a 'reimagining' of the entire series (while general ideas of the previous games exist as canon, they occurred very differently in the reboot universe, the classic games themselves exist more as 'fanciful' stories told by parents to their children in this world, but aren't accurate historical events). Initially it was believed the episodic game was neither a sequel nor a reboot, which told a number of stories that take place in between and after the original series. Sierra/Activision confirmed it to be a reboot. As it was always planned that the new KQ (whoever made it) would be a reimagining they knew it couldn't be the true King's Quest IX.
- “It’s going to be hard to make a King’s Quest IX because everyone’s so attached. The directive was to do a reimagining anyway."
- The game is completely reimagined; it’s not King’s Quest IX or anything like that,” Korba explains. “Think about it like when someone reimagines The Wizard of Oz, or Peter Pan. That’s how I see King’s Quest, [as one of] those classic fairy tales that keep being retold and reimagined.”
- That's also part of the reason the game is a brand new story, instead of a continuation called King's Quest 9 — the developers still have the burden of living up to the King's Quest name, but they don't have to follow the exact path laid down by the eight previous titles.
It is worth noting that from the series perspective see KQGS timeline that King's Quest IX must take place when Graham was in his 70s or older. The 'present' in KQAOG, is when Graham is in his 90s and developers have said that its set about 20 years after the previous games (both in development and roughly chronologically) so about 20 years after KQ8. The series went on to more or less retell aspects of KQ1 (KQC1 prologue), KQ2 (KQC2), KQ3 (KQC4), and hint at or describe details form KQ5 and KQ6 as well. Many of these completely different than their original counter parts. Hence the 'reimagining'. KQ2 is completely retold in a new manner in Chapter 3, and involves two Valanices, and a kinder/misunderstood Hagatha. KQ3 is both partially covered and hinted at in Chapter 4 in as much that it shows Alexander's kidnapping, and his return to Daventry. The three-headed dragon and saving Rosella did not happen in this universe, Alexander's motive for returning home was different. Details of KQ5 include that Graham already knew about "Icebella" before the events of the game (though he only directly met a previous Icebella, but he had met Ice Guardian who would become the new Icebella, and he had previously known Cedric as well), and he while he defeated Mordack he believed he had survived the battle by hiding under the floor boards (in KQ5 Graham believes he killed Mordack, and there was only hard stone where he died). Other things like Cedric's backstory, Graham's backstory, geography of the world is different as well. Etheria is a nearby land to Daventry rather than existing in a separate realm of fairies.
Behind the scenes
There have been a number of attempts to either make a King's Quest IX or series 'revival' or 'reimagining'. But most were cancelled. The Odd Gentlemen's series is the first to actually make it nearly twenty years (since 1998). An alternative way of looking at things would be to consider Chapter I: A Knight to Remember as KQ9, ch2 as KQX, ch3, as KQXI, etc. But this might add more confusion. It's also possible to see Chapter I, as "KQ1" of the new series, etc. But to avoid confusion it will be counted as 'KQC1' instead. King's Quest IX is not The Silver Lining.
- According to Cindy Vanous;
- 9 was not in the title, because Marketing decreed that several other big companies' series were no longer including installment numbers, so we should follow the naming trend.pers.comm. 10/11/2020
- Ken Williams, Not All Fairy Tales Have Happy Endings, pg 358: "I should probably mention that Roberta and I turned over all of our rights to everything associated with Sierra, to CUC, when we sold the company. The King’s Quest brand and all of the characters were now the property of CUC. Roberta could not do a King’s Quest game for anyone else. In fact, she couldn’t work for anyone else under any circumstance. Both her and I signed non-competition agreements when we sold the company. For some number of years (five, I think) we were prohibited contractually from working on any software that might compete with Sierra."
- ibid. "A compromise was finally reached. Sierra agreed to make some changes to the game (a very short list) and Roberta agreed to take some money and walk away. Sierra was allowed to put her name on the box, and most of the game inside was Roberta’s, but not all."
- Pers.Comm, 10/7/2020
- Ken Williams, Not All Fairy Tales Have Happy Endings, pg 358
- In early 2002, it was announced that the three most prominent adventure franchises in Sierra On-Line's portfolio (King's Quest, Space Quest, and Leisure Suit Larry) would receive new versions that were adapted to the gameplay styles that interested the common gamer, or re-inventions. Like Escape Factory's Space Quest update, King's Quest would have been a cartoon-style action title with very little semblance to the previous titles in the series. Unlike SQ and LSL, KQ thankfully never made it past prototype.) http://web.archive.org/20080205234843/sttnw.blogspot.com/2008/01/kings-quest.html
- One designer (Cindy Vanous) was working for a time at Sierra on an aborted KQ which was for console, mentioned it was headed by Mark. From a comment on Facebook Sierra Gamers, Jan 24th (now removed)
- The Silver Lining (and the letter writing campaign began to save it).
- http://www.gameinformer.com/b/news/archive/2011/12/28/the-lawsuit-between-epic-games-and-silicon-knights-reveals-interesting-developer-projects.aspx Game Informer
- https://web.archive.org/web/20130804034334/http://www.screwattack.com/news/silicon-knights-almost-had-kings-quest Screw Attack
- http://www.escapistmagazine.com/news/view/114960-Epic-Wins-Advantage-in-Too-Human-Lawsuit Lawsuit
- Activision Sierra facebook post