Three-headed Dragon

The Three-headed Dragon was a dragon that ravaged lands across the continent of Daventry and ultimately focused its destruction in the Kingdom of Daventry. It was a massive beast, smoke drifting out of its six nostrils. A corrosive fluid dripped from all its maws and sizzled the earth when it splashed. Its scales were covered with moisture, and where its claws gripped the ground, great gobs of earth were ripped out.

Background
Deep within the forests came rumblings of a terrible beast who was ravaging a bloody trail through the kingdoms of the continent of Serenia, on the way to Daventry. The news of its path of destruction towards the kingdom of Daventry brought fear to land. As the years crept by the notoriety of the beast grew as great as the destruction it wrought, and each time a human was swallowed whole by the vicious beast. Soon the whole population of Daventry tremored with the news of the dragon's approach, and each homestead dwelt in terror.

Time wrought many changes, and with it much sorrow. The kingdom of Daventry was ravaged by the deplorable dragon, and the young Princess Rosella was abducted. The entire kingdom was overcome by the brutal onslaught of the beast, and though forewarned, found themselves helpless to defend against its supernatural strength. Much weeping and wailing was heard throughout the land. Even with its power of prophecy, the Magic Mirror could provide no answers, not even a clue, for some bearer of black magic had cast a cloud of darkness upon its face...

The people of the other lands put up walls surrounding Daventry to contain the dragon there and while those walls saved many lands it amplified the destruction inside of the kingdom. At the suggestion of Gerwain and by the demands of the dragon itself, a maiden was sacrificed to the dragon each year. Still the dragon continued to burn the land all around and drove many people away from the town of Daventry and killed many people. Those who remained were terrified to even come out of their houses. Ultimately Graham was forced to send Rosella to be sacrificed to the dragon, despite the guilt he felt at doing so. She tried to remain brave, hoping her sacrifice would save the kingdom. She was taken to the land of the clouds above Daventry and was tied to a pole to await the coming of the dragon.

Gwydion a young slave boy escaped the clutches of the evil wizard Manannan in Llewdor learned from an oracle that he was the son of royal family of Daventry, and that his sister was to be offered as a sacrifice to the dragon. After crossing the sea to the continent of Daventry, and making his way over the mountains, he finally reached the kingdom of Daventry and rescued the maiden just as the dragon was about to consume her. He cast the Storm Brew spell, the lightning of the storm struck the dragon killing it.

Alexander wrote that the dragon was most likely under some ensorcellement. If it had been marauding on its own, it would have cindered Rosella quickly and moved on to new victims. It is still a mystery to all as to who set that particular dragon of fire to ravage Daventry, as it could neither have been Manannan or Mordack. Although at least Manannan took joy from watching the dragon's destruction from afar.

Behind the scenes
King's quest companion states that neither Manannan nor Mordack could have had anything to do with the dragon's rampage towards Daventry. However, it says nothing about their sister, Hagatha. It may be possible that the dragon was sent by their sister in revenge for losing Valanice. However, the dragon is sentient enough, that it is capable of demanding specific maidens (see Rosella), that it's possible that it made its journey on its own, and made its own decisions. Or at least only partially in control of its own decisions, and ensorcellement stayed its breath.

In both KQ3 and the King's Quest Companion, its stated that the dragon demanded the sacrifice of Rosella specifically (she was taken up to the land of the clouds and tied up for the dragon). . Strangely enough, in some of the synopses published in other sources (KQ4 manual, for example) it is suggested that the dragon "abducted" Rosella from Castle Daventry.

Its not specific when the dragon finally reached Daventry in most of the sources (and the amount of time varies).

In KQ3 says it reached Daventry years ago. It is implied by Rosella that she remembers a time when Daventry was still beautiful and Castle Daventry was in good condition. Which would suggest she was at least a young girl when it happened.

According to Roberta Williams in the The World of King's Quest: A Brief History of Daventry, its suggested that the dragon reached Daventry, about 18 years before KQ3, not long after Alexander's kidnapping. This account would suggest, that Graham sacrificed as many as eighteen maidens.

The KQ4 manual suggests that it was years before it finally reached Daventry, and that Daventry was hearing stories of its destruction in other lands. It suggests that the Dragon reached Daventry about the same time, the magic mirror stopped working(in KQ3 this was on the night of Alexander's kidnapping), which would also seem to corroborate with Roberta's point of view.

According to the King's Quest Companion it took years for the dragon to finally reach Daventry (but doesn't explain when it reached exactly), but the description seems to paraphrase the description given in the KQ4 manual in part. However, it seems to place the dragon's date of arrival at least several years after his kidnapping (as it describes Rosella's early years as fairly uneventful, including her education, and time spent swimming in Daventry's many lakes, and that she had 'happy times of childhood' ).

If King's Quest novels are taken into account, it would have to have occurred after See No Weevil (as things are relatively peaceful in the novel, all though Daventry is partially ravaged by the weevils and sloks). That would mean approximately 3-4 girls (including Rosella) were sacrificed to the dragon.

Three-headed dragon (unofficial)
Three-headed dragon (unofficial)

Three-headed dragon (unofficial)
The three-headed dragon has appeared or is mentioned in fan fiction, including fan games, see three-headed dragon (unofficial).