For Sarin’s skeleton in Baldur’s Gate 3, you need to place Sarin’s Skull into the coffin prompt—nothing else will work.

What to put in Sarin’s skeleton (BG3)

  • The correct item is the unique head item “Sarin’s Skull.”
  • When you interact with the coffin/skeleton, choose Sarin’s Skull from your inventory to insert it in the empty head slot.
  • Doing this completes the mini‑puzzle and rewards you with the Chancer’s Carcanet , an amulet that lets you use a special Reaction to gain Advantage on an attack roll or saving throw once per long rest.

Quick how-to context

  • You find Sarin’s headless skeleton in the Lower City Sewers / Undercity Ruins in Act 3.
  • The skull itself is in a pouch behind the silent monk / Voiceless Penitent Bareki near the Undercity Ruins waypoint; you usually need to sneak or use invisibility to steal it.

So if you’re stuck at the “Insert item” prompt: scroll your inventory and select Sarin’s Skull —that’s the only thing you’re meant to put in Sarin’s skeleton.

TL;DR: For “what to put in Sarin(a) skeleton BG3,” the answer is: put Sarin’s Skull in the coffin to solve the puzzle and get the Chancer’s Carcanet.

Information gathered from public forums or data available on the internet and portrayed here.