A Wahrk is a type of giant fish-like creature native to Riven. They are mainly found in the waters of Plateau Island and can be seen with Gehn's underwater telescope. Gehn uses the "Great Whark" as a symbol of his power. It induces superstituous fear to the Rivenese.
Wahrk statues with offerings are found in the Temple of the Dome Island, and crude totems are found in the lake of the Plateau Island. Their bones and skulls are used for constructions, such as the Wahrk Gallows.
It is also stated by Cyan Worlds that the telescope on Plateau Island doesn't line up with the spinning dome, due to an angry Wahrk ramming into it.
Wahrk bones can be seen in the New Mexico desert, suggesting one of them was sucked into the Star Fissure when Riven was destroyed.