Mol'maroch The Nethershaper

Mol'maroch the Nethershaper was the strongest of the three betrayer gods (the Three Dooms) of The Calamity, all three of whom took part in the betrayal of The Harsh. He rose to his power through fiendish collaborations, and the dragons of The Chromatic Conclave served him. Binding him "required manipulating reality, for an end to death". At the last battle, in the mountains of what is now Creatis, Herentor turned him to obsidian with true polymorph, and Kilseth shattered him.

His remains are still a prisoner of Creatis:

• The condensed obsidian block, levitating with pieces missing, sits under heavy guard in the Shrine of the Last in Fort Obsidian.
• Fragments scattered across the world are the Shards of Obsidian, which Heishen the Deathless has spent generations collecting. Heishen's contract with The King In Yellow granted him the power to absorb the power of "the Betrayer god Mal Moroch".

In campaign 3 the party learned that the Divine Gate stays anchored "because the last god was turned into obsidian and is spread across the galaxy".

Campaign 1 touches him too. The Fatemakers' enemy Sim (Simeria) may have known the Nethershaper intimately. Before the March 2020 flashback (real time), an orange- and green-eyed being offered to show the party the Nethershaper.

The Fatemakers' notes knew only that he too was trapped, they weren't sure where.