Ularan claimed that Jergal has been divesting himself of his domain over death for a reason.
Just like The Dead Three he had granted divinity to other mortals over the millennia of his existence, collecting that power when these new gods inevitably died, either from not knowing how to handle their power or being too ambitious with it.
Each time the domain of death has been in the hands of a God who died it is empowered by the event, the death of a God being an event of significant power.
Ularan claims that when the domain has been sufficiently empowered, Jergal plans to snuff out all life and bring the End of Everything.
It is unclear why Jergal is planning this, how long until he is successful, and if Ularan Mortus is even correct about this.
This seems a far too large problem for any mortal to even think about.