In Burning Wheel, that would be the result of a failure - like the actual twist I've posted about a few times now, namely, Thurgon finding letters that seem to indicate he is the grandson of the evil wizard Evard.And this brings up my questions about horror, mysteries, and twists.
What if the purpose (or one possible purpose) of the spellbooks (or some other item) is to provide a twist in the game?
In my Torchbearer game, one of the PCs found a gem, and failed in a spiritual conflict with the dream-spirit that inhabits it. Hence she is cursed by the gem (somewhat like Gollum and the Ring). Mechanically, the way this works is that it requires the player to write a particular Belief for their PC, namely, about hoarding the ring, protecting it from being stolen, etc. Thematically, the cursed gem was not a MacGuffin that (in thematic terms) comes from nowhere: being an Elfstone housing the dream spirit of an outcast Dwarf, it clearly resonated with the two PCs, and Elven Dreamwalker and a Dwarven Outcast.Here's this MacGuffin that other people are going to want to use, possibly for harm. Do the players keep a hold on it? Try to destroy it? Use it for themselves? They may never have thought to even look for the MacGuffin, but here it is.
In a Burning Wheel game that I GMed, one of the PCs was dominated by a Dark Naga. (In BW the spell is called Force of Will.) The descriptive wording of that spell is that "The caster’s words become thoughts, embedded and resonating against the victim’s personality for the rest of his/her days, as if the victim had formulated them him-/ herself". Mechanically, the way I resolved this was to require the payer of that PC to rewrite one of his Beliefs as being oriented towards service of the Dark Naga. (We worked out the details in a conversation that took probably five or so minutes.) I was quite pleased to see that my approach later became official, in that a reprinting of the rules a few years later adopted the same mechanical implementation of the spell (which, earlier, had been silent on how it worked mechanically). This reinforced my confidence that I have a good technical grasp of the game as intended by its designers.
A similar mechanical technique could be used to emulate, for instance, Raistlin's possession by the spirit of Fistandantilus. This is a strong element of this family of games, permitting classic tropes like curses and possession to be part of the game, without the upshot being that the GM tells the player how to play their PC.
Thurgon drove of a demon outside Evard's tower, by force of arms. As a result, he earned an infamous reputation in the Hells, as an intransigent demon foe. Reputations are one element of PC build that can be used to represent the meaning to others of some fact about a PC, or some event in their life. Is that the sort of thing you mean?There was an example of Thurgon's grandpa being a demon. Does this mean anything in the greater world now? Or is it just a personal torment that has no bearing on anything unless the PCs bring it up again. Can the GM have this new fact affect Thurgon in any way?