Sitting here bored (I currently have a broken leg... sitting isn't optional) I thought I'd flesh out the idea I started on above.
World wonder option for Infernal Grimoire appears at the same time as Infernals, who cannot then build it. When it's built it gives 1 or even 2 free techs (one tech not being a prereq for 2nd) and infernals cannot declare war on the possessing civ. Possibly also an alignment change, one step towards evil. It must be carried at all times (possibly only a disciple unit) but can be passed to others. The current owner cannot enter lake, mountains or be carried by a transport, (To stop it from being sailed around the North pole).
But Inf Grim is a curse, worth getting for the techs, and protection against a potentially raging Hyb, but each turn there is a chance (small), that one of your units will be subverted and given a hidden promotion "watcher". Loyalty would help prevent this but should not be 100%. The unit functions and behaves normally, with no external sign to the owning player. Slowly, the longer the Inf Grim is owned, the more watchers are subverted. Infernal player can see everything the watcher sees.
Should the Inf Grim be brought within say, 2 squares of a watcher, the infernal player has an option to "grab" using a world spell mechanism. at this point all watchers become HN and can freely attack the carrier, should they succeed, they now possess the grimoire and must take it to infernal cultural borders (or possibly Hyb himself). Once this criteria is achieved, Infernals Will declare war on previous owner, for having the audacity to summon it in the first place.
Infernal Possesion of Grim allows any of above stated benefits. Note this mechanism may also be used as a way of raising hyborem again should he be dead.
It's probably got huge AI holes in it, I just thought it sounded fun!