Behold, envision a figure of unparalleled greatness, a majestic legend whose name echoes through the ag- Ok I'm doing too much.
So, Imagine the king is an absolute legend of a hero, and people think he's ruling over the land but he's been dead for 50 years and no one knows, and people create a Mythos about him, saying things he never actually did to want to look cool and say they saw him
As I understand it - Basically it's assumed he is kept barely alive to keep the astronomicon (the beacon for warp travel) working. Some people think he is a perpetual (someone who can revive after death) but if he leaves the throne the warp in Terra won't be kept in check anymore and will be lost. Most likely they'd lose the ability to travel the warp as well.