Unstable Stone is in a chest west of the Castigator’s Keep Beacon, on the lower wooden platforms around the stone pillars — hit a fake wall at the second pillar on the north side to reveal it.
Unstable Stone is one of the Combat Tarstones you slot into a sidearm, and it turns steady fire into an explosion once you land enough hits on the same enemy. It sits behind a fake wall near Castigator’s Keep, so you can walk right past it if you do not know to swing at the stonework. Here is exactly where the chest is, what the Tarstone does, and when it actually earns its slot.
Where to find Unstable Stone in Mortal Shell 2
Start at the Castigator’s Keep Beacon and head west. Rather than staying on the upper walkway, you want to drop down to the lower level below the Beacon.
Down there you will find a set of wooden platforms laid out around several stone pillars. Move onto the platforms and work your way toward the pillars on the north side of the area.
Key Stop at the second pillar on the north side and hit the wall next to it. It is a fake wall that disappears when struck, opening up the chest that holds Unstable Stone. Loot it once and the Tarstone is added to your inventory permanently for use with any eligible sidearm.
What Unstable Stone does
| Detail | Value |
|---|---|
| Item type | Combat Tarstone |
| Slot | Sidearm |
| Trigger | Repeated hits on the same enemy |
| Effect | Explosion dealing splash damage |
| Best fit | Rapid-fire / multi-hit sidearms |
Unstable Stone modifies your sidearm so that landing enough shots on a single target sets off a small explosion centered on that enemy, spraying splash damage onto anything nearby. The number that upgrading raises is the splash damage itself — a fixed value per level, not a percentage of your weapon’s base hit — so higher ranks buff the blast rather than your regular projectile.
The one detail that trips people up: only hits on the same enemy count toward the trigger. If you spread your shots across a crowd, you never build to the explosion no matter how many rounds you fire in total. Keep the reticle locked on one target and let the counter climb.
Unstable Stone upgrade levels
| Level | Effect |
|---|---|
| Lv.1 | Explosion triggers after 5 hits on the same enemy |
| Lv.2 | Explosion deals 30 splash damage |
| Lv.3 | Splash damage increases to 40 |
| Lv.4 | Trigger drops to 4 hits on the same enemy |
These are the current Hotfix 2.0 values. Earlier builds listed the stone scaling from 20 to 30 splash damage on a 10-hit trigger, so if you are reading older numbers that is why they do not match — check the in-game description to confirm the figures on your version before you commit points.
When to run Unstable Stone
The stone rewards you for staying on one enemy, so it shines on rapid-fire or multi-hit sidearms and builds that can reliably dump several shots into the same target before it dies or closes the distance. Getting to that fourth or fifth hit is trivial when your fire rate is high.
It falls off if you are constantly swapping targets or leaning on slow, single-shot play, since you rarely reach the trigger before the tempo of the fight moves you elsewhere.
Volatile Fragment and other sidearm picks
Do not confuse this with Volatile Fragment, which is a different Tarstone entirely — it makes enemies detonate on death from ranged attacks, with its own damage values and conditions, rather than building an explosion from repeated hits on a living target.
For the sidearms themselves, the repeated-hit trigger pairs naturally with fast options like the Triarch Repeater, and it also fits the Forgotten Crossbow and Caged Hystrix, all of which can land the consecutive hits Unstable Stone wants.
Keep firing at one enemy — the explosion only counts consecutive hits on the same target, so target-swapping quietly resets your progress toward it.
Video help
Related Mortal Shell 2 guides
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