Collecting and completing all six Serum parts in Ashes of the Damned is one of the most complex sections of the Call of Duty: Black Ops 7 Zombies main easter egg. Between multi-step item requirements, time-gated enemies, and a final chemistry puzzle, this process can feel overwhelming-especially for players attempting the quest solo or for the first time.

The Serum puzzle is mandatory if you want to progress the main easter egg and unlock the DG-2 Turret upgrade for Ol' Tessie, making it a non-negotiable step for completionists. In this guide, we'll walk you through all six Serum part locations and then break down exactly how to solve the Serum puzzle in Yuri's Lab, step by step, so you can finish it quickly and safely.

All Serum Parts Locations in Ashes of the Damned

Serum Part #1 - Powder of the Forgotten

This part requires two key items: Wisp Tea and the Necrofluid Gauntlet.

First, acquire Wisp Tea either directly from Zarya Cosmodrome or for free by completing the Twins easter egg at Vandorn Farm. With Wisp Tea active, head to the Tailor Shop in eastern Ashwood, just right of Vulture Aid. Inside is a mirror that reacts to Wisp Tea. Simply stand near it while Wisp Tea is active to shatter it.

Next, obtain the Necrofluid Gauntlet. Once you have it, return to the broken mirror and shoot it. The Powder of the Forgotten will drop to the floor.

Serum Part #2 - Widow's Lantern

Travel to Blackwater Lake and enter the cabin. Go downstairs into the kitchen and inspect the sink cabinet on the south wall to collect the Jar of Spores.

Bring the jar to Vandorn Farm, just outside the main gate, where you'll find a dead horse covered in mushrooms. Interact with it to plant the spores. After waiting three full rounds, the spores will mature, allowing you to collect the Widow's Lantern.

Serum Part #3 - Mysterious Limb

This is a time-gated drop tied to Zursa, the zombie bear Elite, which spawns from Round 16 onward.

To obtain this part, you must kill Zursa using Ol' Tessie's Abomination Head Beam Attack. First, unlock the Abomination Head upgrade from Reba's Diner at Exit 115 by killing the Cook zombie, retrieving the Freezer Key, and upgrading Ol' Tessie in Ashwood.

Weaken Zursa with normal weapons, then finish her off with the Beam Attack. She will explode and drop the Mysterious Limb.

Serum Part #4 - Ravager Eyes

Ravagers are fast, four-legged enemies that spawn randomly or during Max Ammo rounds. Once one appears, lure it to a Saw Blade trap, such as the one outside the Barn at Vandorn Farm.

Activate the trap for 2,000 Essence and let it kill the Ravager. It will drop the Ravager Eyes automatically.

Serum Part #5 - Hoard Husk Chunks

This part also requires the Abomination Head upgrade for Ol' Tessie.

Once upgraded, drive Ol' Tessie along the interconnecting roads and look for a purple-glowing plant. These spawn randomly. When you find one, use the Beam Attack to destroy it. The Hoard Husk Chunks will drop-just be cautious when exiting Ol' Tessie, as zombies here are extremely aggressive.

Serum Part #6 - Human Bones

Head to the Barn at Vandorn Farm. You'll need a Combat Axe and a Molotov, both of which can be obtained for free. A Combat Axe is found in Reba's Diner kitchen, and Molotovs can be crafted in the Barn or found near Ol' Tessie's garage.

Throw the Combat Axe at the right foot of the hanging corpse in the center of the Barn. Once the foot falls, throw a Molotov at it. After the fire burns out, you can pick up the Human Bones.

How to Solve the Serum Puzzle in Ashes of the Damned

With all six Serum parts collected, travel to Yuri's Lab, located southwest of Zarya Cosmodrome. Inside, you'll see six bowls containing your collected parts and a chemistry station in the center.

Start by interacting with the Powder of the Forgotten (the fifth bowl). Smoke will fill the room, and once it clears, three Pigpen cipher symbols will appear on the chalkboard. These symbols represent the order of bowls you must activate-top symbol first, then middle, then bottom.

Here is the translation, so you don't need to decode anything yourself:

·> = Human Bones (1st bowl)

·L = Hoard Husk Chunks (2nd bowl)

·C = Widow's Lantern (3rd bowl)

·C. = Ravager Eyes (4th bowl)

·L. = Mysterious Limb (6th bowl)

Interact with the correct three bowls in the correct sequence. If done properly, the vials will emit red smoke. Interact with the central vial to trade blood and begin the lockdown sequence.

Lockdown Survival Tips

During this lockdown, your health will not regenerate. To heal, you must use the Necrofluid Gauntlet's siphon mechanic by firing and reloading to drain health from zombies. Survive the lockdown, and the Serum puzzle will be completed.

Practicing this entire sequence in Call of Duty Black Ops 7 Bot Lobbies is highly recommended, especially for solo players. Some players even choose to buy CoD Black Ops 7 Bot Lobbies to safely rehearse complex easter egg steps like this without the pressure of public matches.

Final Thoughts

The Serum puzzle is intimidating at first, but once broken down, it becomes one of the most consistent steps in Ashes of the Damned. With this guide, you'll know exactly where to go, what to collect, and how to solve the final puzzle every single run-bringing you one step closer to completing the Black Ops 7 Zombies main easter egg.