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I was playing in ps4 and a trophy says that you have to break a deadly alliance in multiplayer mode, How can I do that?

When you bring up the player/team info in multiplayer (3 or more players in a non-team match), it will let you select a player on your turn to request an alliance with. This lets you share spirit walls and building resources. Since there can be only one winner though, when one of you attacks the other, the alliance is broken.