PlayerUnknown's Battlegrounds developer Bluehole added the team deathmatch-style War Mode to custom games this week, and it plans to roll the mode out to all players as a time-limited event at some point at the future.
In War Mode, squads jump into a small zone on the map with a pre-determined set of weapons and kill everyone in sight. If you die, you respawn by jumping out of the plane again. The aim is to reach a certain number of points before other squads, and you get points for kills, knockdowns, and revives.
Only PUBG partners—certain Twitch streamers, YouTubers and members of the development team—can create custom games, although anyone can play providing they have an invite or the game is made public. War Mode is still in "a very early stage of development", so the custom games will act as a test bed before Bluehole makes changes and rolls it out to the full game as a temporary game mode, as it did with the shotguns and melee-only mode that was added earlier this week.
Custom modes will also be available more widely "at some point in the coming months", Bluehole said on the PUBG forums.