Hidden Power

Ask Druid Acolyte Nessa about the Gemstone.
She is standing right next to Grace O'Malley.

After this you will be sent to Corinth to speak with Saatvik.
You should use your "Recall to Inn", if you want to save some time.

Now you need to talk to Druid Acolyte Nessa, which is also in Corinth by now.
She will sent you to investigate The Bottomless Cup to find the first half of a map.
It will be on the side of a pillar at the end of the broken bridge.

To obtain the rest of the map, investigate Breakwater Bulwark.
Therefor you need to climb up a wall on the left side of the big ancient skull.

As soon as you own the full map, head back to Corinth to meet Druid Acolyte Nessa in tavern.
Remember you Recall to Inn, if you want to save some time.