It's a fantasy role-playing game.

DM: You are Grog Strongjaw, a goliath barbarian . You are the member of the famous group of adventurers. Previously, She  Dimension Door'ed her way into the ritual room, which  Vox Machina  followed over the wall. She seemed to complete the ritual, creating this small, vacuous, spinning black sphere that is locked in place.
DM: If you want to, you can hold. You can use your action to ready an attack so whenever a trigger of your choosing happens, you can then make your attack.
Player: I would like to do that.
DM: Okay. What trigger would you like?
Player: When he reappears.
DM: Okay, and comes within melee range?
Player: Yeah, when I can see him with my beautiful eyes.
DM: Okay, so you're holding your action there. Good to know. Keyleth, you're up.
