Landroval, Horizon Witness

Legendary Creature — Bird Noble

Flying

Whenever two or more creatures you control attack a player, target attacking creature without flying gains flying until end of turn.

Two or more creatures you control must attack the same player in order for Landroval, Horizon Witness's ability to trigger. Creatures you control that attack planeswalkers that player controls or battles that player is protecting won't count toward Landroval, Horizon Witness's trigger condition.
In a multiplayer game, Landroval, Horizon Witness's triggered ability triggers once for each player you attack with at least two creatures.