Book of Thresholds Final Chapter
Okay, so as we know, Yuumi brings Book into every battle. And for the most part, Yuumi uses her own abilities to fight. Because Book is a Book, and is not made for fighting. Now, this has one clear exception, which is Yuumi's ult, Final Chapter, where Book exudes pure magical energy to damage and root enemies. But Book doesn't have any combat capabilities, I don't think. What Book's purpose is, is to open portals to distant places. So that got me thinking about Final Chapter. If it is a portal, like I think, then what place in the world could be so dangerous that mere proximity to it has such an effect? I personally have no idea, but I think it's a fun thing to think about!