Incarnation of The Nameless One can't become a Cleric. However, the Transcendent One does have Cleric levels
(Probably from evil gods, until they too abandoned him) And this stat block is for TNO after the merging (though the abandonment would remain and the levels would revert to Ex-Cleric, as seen here).
I do still have a bit of tinkering to do (nothing major, however). The thing I'm really worried about is the CR. Every CR calculator I've used (including Tribble's) completely fails. I decided on 25 because the physical capabilities (save for blind spots like AC and pre-power attack damage) are near the Sirrush, and TNO has the additional benefits of Wizard and Rogue class features.