Kyuss's ascension clearly didn't go as he planned, but why? My players are dead-set on discovering this, and it makes sense--the information could help them stop him, and also, one of the PCs is plotting her own ascension and doesn't want to screw up the way Kyuss did.
I can't find anything about this in the modules.
Has anyone come up with a clever answer? I'm currently thinking that there may have been bad blood between Kyuss and Nerull (there's a reason that Nerull isn't part of the Ebon Triad....) but I'm very open to other answers.
"The Harbinger of Worms probably knew this," say the PCs. "Too bad he's dead." And the one PC who knows he isn't dead just pats her backpack, where his phylactery is residing, and smiles to herself.... So they do have the means to find things out, though she is going to have to be very sharp indeed to keep this from the other PCs, or get them to accept it.
Mary
When my party parleyed with the Harbinger before attacking, they asked him why the Rite of Ascension failed. He shrugged with all of his shoulders and told them that he'd been pondering that same question for centuries. His two best guesses -
1) As in your player's hypothesis, Nerull's envy and desire to eliminate a competitor to the title of God of Death got Kyuss trapped in the monolith. If you want to make this a concrete fact, you might want to play up the idea that powerful worshippers of Nerull are preparing for war - Nerullites have seen the signs and are making trouble in Alhaster, Nerrulite bodies are found as zombie banquets in the Writhing Tabernacle, maybe even add a few erstaz Nerullite allies in "Dawn of a New Age".
2) The use of the Worm in the Rite of Ascension was both the plan's strength and its greatest weakness. Despite its power to focus the devotion of Kyuss' followers into a single nexus point, the Worm's inherent life in the portfolio of Undeath caused enough of a flaw in the procedure to render Kyuss divine, but impotent.
My personal opinion is that the ritual failed because the Harbinger intended it to fail.
He taught Kyuss just enough to let Kyuss do the ritual the way he did, entrapping himself in the monolith. The Harbinger then waited as a lich until the timing was right, at which point some nice, helpful adventurers came along and destroyed the obsidian ring, killing the Harbinger's body and thus putting him into his phylactery in the process. (In my version, the phylactery wasn't on the site of the Spire, but it can work either way.)
Yup, everything is proceeding exactly as he planned...