Multiverse

Mentioned in King's Quest Companion, the Multiverse is the combined realms of the world of Daventry, Earth, Eldritch, and other planes of existance.

Derek Karlvaegen and others would send messages across the multiverse to either Peter Spear, Eluki Bes Shahar, Jane Jenson, Roberta Williams and other people either through their computers, dreams or visions. Then Peter Spear published these messages as the King's Quest Companion to help people complete the games, and also tell people "what really happened".

Apparently space quest, quest for glory, king's quest, and other sierra universes are connected through the so-called Multiverse. Peter Spear for example also claimed he also received messages from Roger Wilco from the future across the multiverse which he then published as the Space Quest Companion. He makes refrence to his other book in the King's Quest Companion.

A similar concept to withdrawals can be found in the King's Quest ZZT universe, where Jerrod Wilson accidently stumbles into the World of King's Quest from the world of Gold Rush while trying to find his brother Jake Wilson, and has to be told he isn't in his world anymore.

Efreeti are also said to come from another plane of existance.

Dreamland exists on another plane of existance.

The three Fates live in another plane of existance as well.

Real World
Creatures from human myths and literature are real in the KQ-multiverse, supposedly originating from our world (so this could be interpreted that KQ-earth is only an alternative version of our Earth, in the same way that all victorian literary characters are real in the League of Extraordinary Gentlemen that takes place in an alternate past).

However it should be noted that conceit behind the story is that the Peter Spear from our earth really received these messages from Daventry, and then sent it to our earth's publishers to be printed for us to read, thus why the book physically exists in our world, and not some alternate universe where we wouldn't actually be able to get a copy.

His conceit behind the story is that we the people of earth really did have a fantastic past, and as time passed, that the modern people of earth who believe in science and technology can not accept our "true history", and are writing revisionist history that denies(though they can't "disprove") what "really happened". Compare to various religions of the world that believe fantastic things of mythic proportion actually happened in the past, and that fantastic beings really existed.

In space quest I its possible for Roger Wilco to end up in Daventry if messes with the controls on his escape pod. In KQ2 graham can look into the realm of space quest if he looks into a hole in a rock. However in the Apple II version you can look into the future and see King's Quest III from the hole. In the unofficial KQ2 (AGDI, hero will come out of the hole to plug AGDI's unofficial Quest for Glory II remake. These are not indicative of the existence of the Sierra-multiverse though, and are deemed simply as easter eggs and injokes.

The Peter Spear connections need a bit of clarification. Spear did indeed receive communications from Derek Karlavaegen in Daventry which were included in his King's Quest Companion. However, the Space Quest/Roger Wilco material was left on the doorstep of the Sierra On-Line offices in California in the form of a number of computer disks. They were discovered by John Williams (the brother-in-law of Roberta Williams) who was of the initial opinion that they were a hoax. Later events convinced him otherwise and he made the disks available to Spear for editing and publication a few years later. Spear was chosen specifically because of his King's Quest work.