Stonecold Stronghold is a perk that returns from Black Ops 4, and on the Rex Infernus map in Black Ops 7 Zombies you earn it as a wearable skull mask instead of a Perk-a-Cola. The mask sits inside a hidden wall safe in the starting house, and the safe only hands it over if you open it during the right rounds.
Quick answer: Melee the landscape painting on the bottom floor of Her House to slide it open and reveal a wall safe, then open that safe on Round 9 or 10 to get the golden skull mask. Equip the mask to gain the Stonecold Stronghold perk.
Where the hidden safe is in Her House
Every Rex Infernus run begins inside the house area, where you hold out against zombies before the main quest opens up. The safe you need is on the bottom floor, tucked behind a landscape painting to the left of the exit. Nothing marks it, so you have to know it is there.
Meleeing the painting slides it aside and exposes the wall safe behind it. The safe itself is always in the same spot, but what it gives you is tied entirely to the round you open it on.
Safe rewards by round
The safe scales its contents with your round count. Open it too early and you get nothing, so it is worth waiting until the window that drops the mask.
| Round opened | Reward |
|---|---|
| Before Round 5 | Nothing (empty safe) |
| Round 5 or 6 | Ammo or Essence (+600 Essence) |
| Round 7 or 8 | A random perk bottle (such as Elemental Pop) or a Blue (Rare) Aether Tool |
| Round 9 or 10 | The golden skull mask |
How to unlock the skull mask
What Stonecold Stronghold does
The perk rewards you for holding your ground. Stand completely still for a few seconds and a purple ring of fire forms around you. While it is active, your armor regenerates and any zombie that walks into the ring burns.
You know the mask worked when the fire ring appears at your feet after you stop moving and your armor starts refilling on its own. If the ring never forms, you are moving too much, so plant yourself and wait out the short delay.
Fast Round 9 and 10 setup
Because the mask only drops on Round 9 or 10, you want those rounds to go by quickly. Stand in the room that holds the Kuda wall buy. Doing this forces zombies to spawn in just two spots, one in the corner of that room and one in the room next door, which lets you clear each round fast and reach the safe window sooner.
With the painting already opened ahead of time, you can rush the safe the moment you tick over to Round 9, grab the mask, and pair its armor regeneration and fire ring with the rest of your loadout for the harder rounds ahead.
