Quick answer
Start from The Silent Steps in Mammon, defeat Sariel in the open encounter and follow the newly opened route into the Chamber of Becoming. In the final arena, destroy all four glowing stone tablets before the killing blow or Sariel revives. A permanent kill unlocks his Shell and the same route also awards the Clockwork Scythe and Caged Hystrix.
Why Sariel keeps coming back
If you empty the health bar and he stands up again, your damage is not bugged. The final arena has four revival objects/tablets, one around each corner.
Destroy all four first, then finish the boss.
Route from The Silent Steps
- Activate The Silent Steps Beacon.
- Use the nearby traversal/jump gate to reach the higher area.
- Fight Sariel in the open encounter.
- After he falls/retreats, the route behind the large statue opens.
- Enter the Chamber of Becoming.
- Continue until the final arena.
- Break all four glowing tablets.
- Kill Sariel permanently.
Rewards from the same run
The final Sariel route is unusually efficient because several major collectibles are tied together:
- Sariel’s Shell;
- Clockwork Scythe;
- Caged Hystrix sidearm near the exit route.
If you came only for the Shell, do not leave the dungeon before collecting the gear.
How Sariel plays
Full-release Shell comparisons describe him around Curse and Pain. Curse can reflect/prevent the next relevant melee hit from affected targets, while Pain represents recoverable health that can be restored through melee pressure.
That makes Sariel a specialist sustain/control Shell. Launch tier lists have tended to rank him below the explosive boss-control options, but the mechanic can still support a deliberate curse-focused style.