This mission can be completed once you reunite with Rab in Act Two.
Sail to Gondolia and make your way to Hotto via Gallopolis.
Go to the Temple in the North of the town to begin the quest.
I'll not spoil the quest by going into details, but it does involve two separate 'boss' fights in different locations. You can save/rest between both.
They aren't particularly tricky. I managed to complete this with my party around lvl 38-40. A weapon or skill which increases damage to dragons is recommended.
Worth mentioning I completed this before rescuing
as I found that fight too difficult at my current level.