Are you familiar with the concept of adventure hooks? An adventure hook is a small detail that players come across or are introduced to over the course of one adventure that will help transition to the next.

Some possibilities:

1. The players come across some sort of message or communication between the undead elf and the orc chieftain - a letter, a phantom messenger, a holographic communication stand, whatever works.

2. Maybe the orc chieftain had a trusted adviser from before he was corrupted, and this adviser fled the clan when things started going down hill. And this adviser conveniently knows some details about the undead elf and whatever the next phase of the threat is.

3. Maybe some of the souls in the rock were not sacrificed properly, and came unbound from it when the players destroyed the orcs. Now they're following the players, trying to get their help.