So last night my party defeated the evil orc chieftain who had been raiding the trading routes and begun sacrificing his captives on a giant soul sucking rock. The chief was "corrupted" by a wizard who convinced him to abandon his forward thinking ways and start raiding again. When the party stopped the orc chief

What the PCs don't know is that the wizard was part of a cult serving an undead elf who wishes to bathe the world in eternal twilight, no longer restricting her to the frigid north. Anyone slain on the rock has their soul trapped inside it which can then be used for powerful magic. There are four of these stones in the world and enough people must die on them to fuel her spell.

The rock they found was under the village. It was in a small compound that was a spellweaver's laboratory that was sunk under the earth centuries ago. The orcs for the most part were just occupying the place and the majority of the items in the compound belonged to the former spellweaver occupant. His familiar, a fire mephit, waits for his return in his old quarters.

So the problem I'm having as the DM is I've stuck myself in a bit of a corner. I'm struggling with trying to figure out how to go from "Big evil orc killing merchants" to "Holy f*ck! Undead cultists!"

Suggestions?