While Back 4 Blood is definitely a game in its own right, it is always compared to Left 4 Dead due to Turtle Rock Studios’ previous work on this series and the similarities they share. Both have campaigns that focus on a team of four players working and communicating to get past the hordes of undead enemies that stand in their way. Both also feature competitive modes in Versus in Left 4 Dead and Swarm mode in Back 4 Blood. What exactly is swarm mode?
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The swarm mode in Back 4 Blood is the game version of a PvP mode or a competitive mode. Two teams of four players take turns playing as a team of Cleaners and Mutated Ridden to see how long they can survive as humans. The team with the longer survival time wins the round before moving on to the next map. Essentially, you’re playing a competitive timed survival mode.
The experience on the cleaners’ side is pretty standard for the campaign. Before you begin, choose your character and deck of cards to play with. As the round lengthens, a swarm of insects approaches the survivors, forcing them to move into a smaller area until they are eventually killed.
On the Ridden page, you choose which of the special enemies you control. As the round progresses, you gain experience and can pay those points to upgrade your monster or common infected on the field. If you are killed you will be placed on a respawn timer before you can return.
https://www.gamepur.com/guides/what-is-swarm-mode-in-back-4-blood What is the swarm mode in Back 4 Blood?