The Baldur's Gate 3 Sarin skeleton puzzle is one you find while roaming the sewers and cisterns of the city proper in Act 3. In the centre of the underground region, you'll likely bump into Aelis Siryasius and his army of grease creatures. This wizard is on the run from Absolute cultists, and if you can't convince him you're friendly, you'll be fighting for your life against his fatty monsters.
Whether you convince him or not, you'll discover that he's protecting the corpse of his dearly departed comrade, Sarin. If you walk over and inspect the skeleton you might notice that it's missing a skull, and when you activate it, a slot appears into which you can insert an item. So, here's how to solve the Baldur's Gate 3 Sarin skeleton puzzle and find her missing head.
Sarin's Skull location
To fix the skeleton you need to find the Sarin's Skull item—if you gaze into her coffin, you can quite clearly see that her head is missing. Make your way to the Undercity Ruins waypoint, which is directly to the north of where her skeleton is. Once you arrive, head west until you find Voiceless Penitent Bareki, then look for the pouch just up the stairs behind him to find Sarin's Skull. You'll likely have to sneak when stealing this so the vendor doesn't realise.
Now you've got the skull, you can go back to Sarin's coffin. If you managed to convince Aelis Siryasius not to attack you the first time, he will become hostile once you start messing around with Sarin's corpse. Remember, grease is flammable, so fire attacks are a good method to beat him quickly. If you prioritise killing him, the Grease Mephits he's summoned will also perish. I'd recommend fighting him from a distance with fire AoE attacks, rather than starting the fight right in the middle of that greasy hellhole.
When they're all beaten, simply activate Sarin's corpse, and slot in the skull to receive your reward: the Magic Amulet. This gives you the ability to use a Reaction to make an Attack Roll or a Saving Throw with Advantage once per long rest.