High Priest Sylvain:
Rombel! He has extensive knowledge on giants too. He is at Underground Level 3 of Cruma Tower next to Ivory Tower Wizard Janssen.
Then you just need to go to Cruma Tower Underground Level 3 and see Ivory Tower Wizard Rombel. If Janssen referred him, I'm sure he can tell you how to meet the ancient giants.
I hope you learn what the Ertheia need to know. May Einhasad's blessing go with you.