Clerris is an NPC located near the Dense runestone mine.
When players have achieved at least 25% favour in the Arceuus House, they can mine dense essence blocks. Players can hand these blocks to Clerris, rewarding 0.1% favour for every block handed in.
Once players have achieved 100% favour in the Arceuus House, Clerris will tell them that she does not require any further dark essence at present.
Clerris is the solution to the medium anagram clue: R SLICER, with the challenge scroll answer of 738.