The half-covered well in Cassardis is one entrance to the Dripstone Cave.
The other can be reached shortly after going beyond the encampment where Pawns call through a riftstone to the Arisen. With Cassardis behind, pass the encampment and take the left path as soon as it appears. A few steps down the path there is a door in the cliff. That is the other entrance to Dripstone.
Zoning in from either end will cause this achievement to appear.