Aarakocra Sneak

If the player with the initiative leaves the game, the active player takes the initiative at the same time that player leaves the game. If the active player is leaving the game or if there is no active player, the next player in turn order takes the initiative.

A player who currently has the initiative may take the initiative again. This causes that player to venture into Undercity again, but does not cause them to have multiple initiative designations.

Only one player can have the initiative at a time. As one player takes the initiative, any other player that had the initiative ceases to have it.

You cannot venture into Undercity unless instructed to do so, either because you have the initiative at the beginning of your upkeep or because you take the initiative. Notably, if you aren't in a dungeon and an effect instructs you to venture into the dungeon (not venture into Undercity), you can't start Undercity.

Similarly, when instructed to venture into Undercity, you can't start a dungeon that isn't Undercity.

If you aren't in a dungeon when instructed to venture into Undercity, you will put Undercity into the command zone and move your venture marker to Secret Entrance (the first room).

If you're already in a dungeon when instructed to venture into Undercity, you move to the next room of that dungeon. If you are already in the last room, you will complete that dungeon and start Undercity. This is true whether you're already in Undercity or any other dungeon.