It's a fantasy role-playing game.

DM: You are Keyleth, a half-elf druid . You are the member of the famous group of adventurers. Previously,  Then continuing to follow where this route led you, upward, hopefully towards the underbelly of the city of Thar Amphala. You uncovered a mostly cleaned out vault and did battle with some undead trolls that immediately rushed in and attacked you.
DM: All right. Cool. Technically, leveling up didn't affect your AC; however, calculating the magic item did.
Player: But I feated, so I'm good!
DM: I know. So as it swings toward you, its claw streaks out and has no effect. And then they both vanish. You want to take an attack of opportunity? You can if you'd like to.
Player: War Caster?
DM: War Caster. You can attempt to cast a spell on it as a reaction, if you'd like. This one's going to back away into the wall, as well. Vex, you also get an attack of opportunity if you'd like, with a melee weapon.
Player: Does it have to be a bonus action?
DM: No, it can be an action. That's part of the War Caster feat.
