How do I defeat the Spawn of Rovagug at the bottom of the Tenebrous Depths?

I’ve done it! I’ve cleared all 16 levels of the Tenebrous Depths and revealed the true nature of that miserable dragon back at the entrance that sent me in there. He teleported us down to confront the monstrous beast below the dungeon and…

Promptly turned on me. That Jerk.

So now I’ve got to deal with fighting a monstrous demigod, and his little friend. And I’m having no luck here. What am I missing? What debuffs or control options, if any, will work here? What damage types should or party members should I be taking?

My player character, if it matters, is a Sword Saint Magus built primarily to tank (~60 AC, but very high single target damage with buffs like legendary proportions and some crits), and I usually use something like Jubilost, Nok Nok, Octavia, Tristan, and Valerie, but I’m open to being told some other party member or build or class skill is essential to making this work. I’m not quite into the endgame, but with level 17 characters I’m pretty close to it and should be working with a full deck of cards at this point if I prepare.

Solution 1:

I have faced the same problem as yours (with the same character) and maybe I've just been lucky but I have beaten him quite easily, despite losing a couple turns and having several characters be less than useful in that fight.

I didn't focus much on buffing since the spawn has a dispel attack.

I rushed the

dragon,

aiming at killing them as fast as possible. Sword Saint Magus, either tank Valerie or Amiri (can't recall) and Ekun focused fire on them, while Octavia cast sneak-attack AoEs on everything hostile and Kaessi created a mud deadly earth below the spawn, a little behind and to the right. Jubilost stood to the side, waiting to be able to focus fire on the spawn (there's not infinite bombs, using them just to deal a little damage that the spawn easily heals is not useful at all) but if you need him for taking out the other enemy in one torn, so be it.

Then, I placed the other melee in front of the spawn, my Magus at the top (in order to force the spawn to only target one at a time with its cone attack) and kept unleashing arrows, bombs, fire rays and mud long range kinetic blasts (which almost always missed) in the Spawn. Ekun got possessed but he had the freezing bow and targeted Jubilost, who had good cold resistance.

Jubilost used force bombs, because I had not taken holy bombs. Holy bombs work better, as does Smite if you happen to have a paladin available.