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.

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Damn I got really excited about this idea and then your comment made me realize all the players at my table know Warhammer better than I do and would get it immediately