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, Unfortunately, their actions attract the attention of the undead dragon they saw previously, which comes to investigate. They hide in a nearby building, but Delilah and Sylas, on the dragon's back, find Arkhan hiding under his gloomstalker.
DM: Okay. At the end of your turn, Sylas is going to use his last legendary action to move his speed without provoking opportunity attacks. He leaps off the side and lands right next to Delilah.
Player: Does he get entangled? Because he did just land in difficult terrain.
DM: He's not entangled, I don't think, because he wasn't there for the casting. Difficult terrain means he moves across at half.
Player: He's in difficult terrain.
DM: Okay. Good to know.
Player: I have rocky ground. That's all I have.
DM: All righty, so entangled... okay, yeah. Got it. Cool. That ends the legendary action. Grog, you're up.
