The Tomb of Yvenia is near Fort Tarsis but is a little difficult to find and can be missed easily. Moreover, you have to reach a certain level in the Main Story to get to this challenge.
From the launching platform of Fort Tarsis, start walking left following the wall. You will see another way at the end that will lead you to a stream. From south of the stream, you will head into a cave which will take you to a chamber.
You will see a glowing door. That is your Tomb of Yvenia. It is on the ground so can be missed if you are flying. Hopefully, this guide will take you to the door of the Tomb.