A Key Event

They're TONS of key events, FAR too many to name here reasonably, so instead of naming each individual one, here, I'll list what KINDS of key events there are.

The Lore Quests
These quests are MANDATORY. To continue the journey through the Underground, you MUST complete each lore quest, or you'll be stuck forever. Besides, why pick a story if you aren't going to listen to it?

The Character Quests
Each main character and a few side characters have their own questlines, with even some secret questlines here and there. Unlike the Lore Quests, which tell generic lore, Character Quests tell lore about that specific character. Certain character quests are only allowed on certain routes you take.

Side Quests
Side Quests are completely optional, and usually a bit of fun. There are no real rewards for these quests. None, except new friendship anyways.