Garen's Magic

It is in no way confirmed.

However, looking at the visuals and the reveal that the petricite absorbs magic. I don't think it's far off to assume that the perspective of Garen is going to change. A year and some ago, a Rioter claimed that Garen's ult was not going to be represented in his lore in any way, simply calling it just a gameplay feature.

It now seems set up that Garen's sword has absorbed the magic from all the mages he's fought or killed, and he'll probably be able to channel that power into his ult in lore. Or at least, that's what I think they're going to do. I have no proof though.

Yes, according to Swain.

"Ignorant of the very magic you carry into battle. How Demacian."