Finish a quest for Merana and turn one of your pets Hydrus! 0/1

where do we go to do this quest, or is it just random?

You can do this quest in Omen Island. To be able to go there you need to complete a Uncharted Map first, though.

Good Luck..

Visit Merana in the omen islands, pick which pet you want to turn hydrus and she'll give you a list of items you need to find for her.