The Cta-Tha is a large Draug lord, similar to the Varangian from Polaris. Agetor notes:
Mainly use a poisonous cloud AoE that he use regularly, has to be moved out of it has it cannot be interrupted [Note - this is incorrect, it can be impaired]. Otherwise an easy fight. About 175 000 HP
The poison cloud that Agetor references is an ability called Tidal Brine. It can be interrupted, via impair, meaning you can bypass that ability. Cta-Tha also has a wide angle front attack, but it is easily avoided.
Summoning Fragments for Cta-Tha