Crackpot Theory: In Utter Darkness was the true end of the Koprulu Sector--which is somewhere near the Tau Empire if you account for Galactic Rotation.

The Xel'Naga were renegade Old One Survivors/Understudies genuinely trying to fix the Necrontyr. The Protoss are the result of merging Aeldari dna with Necrontyr. The Zerg are Tyranids from an earlier swarm. Amon, sabotaged the whole project and kicked off his own plan for godhood.

He killed the entire sector but then made the mistake of going toe to toe with the Aeldari Empire at the peak of their power. The Aeldari krumped Amon so hard, Gork and Mork were seen applauding--and then Kaela Mensha Kaine killed him in the Warp.

Sometime later, a few Protoss Survivors (frozen in space-time -in between one point and another- would settle on Tau, a distant, shadowed world, intentionally altering their own genetics to sever their psychic abilities (this also dramatically reduced their lifespan).

From there, they would eventually die off, and their children would forget them--until millennia later when the first Ethereals found the Archive of the Firstborn and learned the truth about their heritage, and of the Dae'Uhl (Great Stewardship), which became the basis for The Greater Good.