726.2 There are three inherent triggered abilities associated with having the initiative. These triggered abilities have no source and are controlled by the player who had the initiative at the time the abilities triggered. This is an exception to rule 113.8. The full text of these abilities are “At the beginning of the upkeep of the player who has the initiative, that player ventures into Undercity,” “Whenever one or more creatures a player controls deal combat damage to the player who has the initiative, the controller of those creatures takes the initiative,” and “Whenever a player takes the initiative, that player ventures into Undercity.” See rule 701.49, “Venture into the Dungeon.”
726.4 If the player who has 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.