A part of the on-going partnership between Activision and Sony, which includes offering timed exclusive content on PlayStation platforms first for multiplatform games, Onslaught is basically a horde mode where you and one other player need to survive increasingly difficult waves of zombies.
Maps will bring the zombie curse to a wide variety of familiar locations from the game - this probably means that maps from a regular horde mode are going to be recycled - for this 2-player co-op mode. Custom Loadouts will be used here just as in Multiplayer and other Zombies modes. Movement will be restricted at the start of the match by a Dark Aether Orb, so you’ll need to find a good place to hunker down within that restricted area. Beyond the borders of the Orb you will continually take Dark Aether damage. As you slay more and more undead, the Orb will be powered up and move - making more resources available.
Each time the Orb powers up, it will release a surge increasing the power of the subsequent waves and spawning elite enemies. These should be your main targets, not only because of the threat they pose but because they greatly influence the score you will receive when the game is over. Onslaught will include special unlocks which you cannot get in any other mode, which might end up giving PlayStation players an advantage in cross-play modes - until the 1st of November, 2021 when Onslaught ceases to be an exclusive mode. Call of Duty: Black Ops Cold War will launch on the 13th of November, 2020.