(1) All major players in Raven's purge can be killed somehow
Of the major players in Raven's Purge, most are at the level the PCs should be at the end of the campaign so don't need weaknesses. Zertorme and Zytera will go mad if they put on Stanengist (so will Merigall, but he knows not to; and Krasylla, but he doesn't want to). Zytera can also be cut into bits by Asina, and if you have a lock of Zygofer's hair you can summon vengeful demons.
Krasylla can be shot by one of the arrows of the Fire Wyrm, and is bound to Zygofer's service by a contract he signed in blood that Zygofer has on him (quite why Krasylla hasn't tried to steal it off him is interesting, but maybe the contract explicitly binds him from even thinking about stealing it?).
Merigall's life essence is inside a stone statue, and if you can get either really a whole lot of salt water, or some acid and some lye, you can kill him.
(2) Merigall's life essence is neither difficult to get nor a threat to him
You can get hold of Merigall's life essence either by straight up defeating the stone guardians in combat, throwing Ivelde into the room and waiting, or wearing a silver hauberk and examining the guardians carefully. You might have to smash up a few statues to find it, but all you need for this is a sledgehammer.
It is completely out of character that Merigall, who has walked through this chamber many, many times, has not somehow sensed that his life essence is in this room, or overheard Zygofer gloating about it, or knows for some other reason. This is literally his number one goal!
Also, if Zytera decides that Merigall needs to die right now, he needs to go to the guardian chamber, find the right statue, get the brass receptacle out of it (either by smashing it or maybe there's a hidden catch or something), then either take it to the ocean (which most people don't put Vond anywhere near), or do some basic but slightly time-consuming chemical stirring.
This is not an instantaneous process, and you'd think that someone who lives next door to you would notice you doing this, and would wonder whether he should use the weapon of demon-slicing that he happens to have in his bedroom, against you.
(3) A smart Zytera would have a better threat
At the very least, if Merigall tries to kill Zytera, he should also die.
One way might for Merigall's life essence to be inside Zytera's spider demon body, separated from but very near a mixture of acid and lye. If Zytera is killed, that separating membrane will dissolve and Merigall will start to die. Apart from the perverse incentive of making Merigall want to dissect Zytera's body if he realises this, though, there's no guarantee that the life essence will dissolve in time.
No, it's better if Merigall's life essence is somewhere that Zytera knows about but Merigall doesn't, in the hands of trusted henchpeople, who could empty it into the sea at a moment's notice. This probably means some isolated semi-monastic community living on top of a precarious pillar of rock joined to the mainline only by a rickety rope bridge, but they could also be in a small village of cliff-face dwellings lashed by gale-force winds, any kind of coastal port village where acolytes could venture into the harbour and empty out a ceremonial urn at the dead of night, or even somewhere inland where an underground river briefly surfaces before it makes its swift way into the ocean many miles away.
For best results, have a few of these, only one of which contains Merigall's actual life essence, to keep him guessing.
The trigger might be (a) Therania explicitly telling them "your sacred duty now needs to be fulfilled" or (b) her having told them "if I haven't visited you for some period, I have died, and you need to avenge me". Zygofer grudgingly admits that her way is better, but out of insurance he'll have also have Soul Bound a few demons who will have been "interred" in sarcophagi; when he breaks some rings on his fingers or otherwise frees their spirits, he hopes that they'll go on a roaring rampage of revenge that results in the temple complex falling into the sea, Merigall's life essence with it.
Maybe you still go with the charade that Merigall's life essence is in one of the statues (for added verisimilitude, maybe it was once, before Zytera changed their mind). Either way, it keeps Merigall guessing.
And it means the players can potentially luck into having Merigall's life essence before they storm Vond with an army.