It's deliberately somewhat obscure, so I'm tempted not to spoil it.
But since it's not clear enough, what the heck: the speaker(s) are the predecessors/slavemasters of the baernoloths on the Waste ("the only place of power"), the ones that the baernoloths usurped to claim the title of Exemplars Of Evil. Where the baernoloths represent the evangelism of Evil and the desire to destroy Good, their predecessors were the simple of Evil of selfishness and vice ("the multiverse [i.e. the Waste -- they're supremely self-centered] existed for such pleasure as we could know."). Ultimately the baernoloths, being more forward-thinking and cunning ("like gluttons at a banquet...", "teaching the wretches without concern..."), rose up against their slavemasters/teachers, and killed them all (obvious quotes there).
That should all be more or less obvious, I hope.
What's much less obvious is what happens next: though their bodies were destroyed, the predecessors didn't exactly die ("But we endure./Long years we waited..."), somehow living on as malign spirits. They bided their time ("Long years...") until they found one or three -- the prayer is deliberately obscure here -- baernoloths to give their ancient mysteries to ("one who was not fit to hear them...") until that/these baernoloths were seduced to the Dark(er) Side, became Apomps, and created the gehreleths ("We were born again.") as vehicles for the spirits of the dead predecessors ("These are not our bodies...").
The upshot? Apomps isn't the god of the gehreleths as much as he/they/it is the slave god of the gehreleths. A patsy they've tricked into giving them flesh until they're ready to strike.
Hope that helps
Roughly speaking, think of each major paragraph as being read aloud by a High Priest (e.g. a Shator) and each single line as being the response from the congregation (the lesser gehreleths). In fact, given the triple nature of the the "endure" line, it should really break into stages: farastu should do the "endure" response, then kelubar should do the next three, then shator should do the next three, with the congregation as a whole reciting the first and last lines.