As a 2-player experience, players will have to survive against Zombies and special enemies on Multiplayer maps.
At the start of Zombies Onslaught, the player will spawn in alongside a Dark Aether Orb, which itself is encased by a glowing, purple dome. Exiting the dome will result in the player(s) taking damage. Zombies will begin to spawn within the Dark Aether Orb-affected region, and must be killed in order to increase the Orb's power to 100% (a bar on the left showcases the Orb's Power, with each kill adding 10% to the progress). Once the Orb Power is at 100%, the Orb will explode and kill all remaining enemies in the area, whilst moving to a different location. This completes what is known as a "Surge". After another Surge is completed, the Orb will move once again and set down, enlarging itself and allowing an Elite enemy (or 2 Special enemies, which are referred to as Elites in Onslaught as well) to spawn, while also counting the Elite spawn by itself as a Surge. Once the enemies are defeated, they will drop a Random Perk Power-Up, as well as various other items, such as weapons or equipment. This completes an "Elite Surge". The Orb will move to a new location, repeating the process indefinitely. Elite Surges also drop Aether Tools, which appear as Dark Aether Wrenches. Picking up one of these Aether Tools will result in the currently equipped weapon being graded by one Quality Tier. Another item that may also drop from Elites are Weapon Chalices, Which depending on the grade, will also provide the currently equipped weapon with a pack a punch upgrade. (Bronze for tier 1 Pack a Punch, Silver for tier 2 and Gold for tier 3). Please note that Weapon Chalices are separate and unrelated to Surge chalices, which are earned through completing a set number of Elite surges in a single game.
Throughout a playthrough of Zombies Onslaught, the player are able to earn "Chalices". These Chalices are granted by completing a certain amount of Elite Surges, and grant XP upon being earned. Bronze Chalices are earned upon completing 3 Elite Surges, and grant 2500 XP. Silver Chalices are earned upon completion of 5 Elite Surges, and grant 5000 XP. Finally, Gold Chalices are earned upon completing 8 Elite Surges, and grant 7500 XP. These chalices, as of Season Two Reloaded, also appear as drops that can be picked up, resulting in the current equipped weapon being upgraded to a higher rarity.
Unlike the typical Zombies experience, Onslaught does not feature the Mystery Box, Perk-a-Cola Machines, or the Pack-a-Punch Machine. Thus, the player must rely on items dropped throughout gameplay by enemies in order to gain new weapons, Perks, and to upgrade their weapon.
Holiday Onslaught is a limited-time mode introduced during Season One of Black Ops Cold War. It is identical in gameplay to the regular Onslaught, albeit with cosmetic, Christmas-themed changes (similar to the ones found in Jingle Hells, as well as piles of useable Snowballs found across the map.
- Main article: Onslaught Containment
Onslaught Mystery Munitions
Onslaught Mystery Munitions is a limited-time mode introduced with Season Three Reloaded. Players would start with a random weapon, with it being replaced with a different, random weapon after every Surge that scales up with the difficulty.
Onslaught Accelerated is a limited-time mode introduced with Season Four Reloaded.
Onslaught Diminishing Light
Onslaught Diminishing Light is a limited-time mode introduced in Season Five. It is identical to the regular Onslaught, besides the fact that the light that surrounds both the player and the Orb gets smaller.
Onslaught Elite is a limited-time mode introduced in Season Six. It acts the same as normal onslaught but an Elite will spawn with every surge. The Elites featured are Megatons, Mimics, Krasny Soldats, Abominations and Manglers.
- Main article: Zombies Onslaught/Intel
Currently, Zombies Onslaught has 57 pieces of intel.