Achievement "Cave in" (25 Ponits)
After getting to Sentinel Island, there is a grave with three candles on the second peninsula to your left. You just need to jump down from the cliff at the end of this peninsula and enter the cave there.
The achievement unlocks as soon as you enter the cave.
You can also fight Adah and Mahalath (both are level 20) in this cave, but this is not required to get the achievement.