He is addicted to Skooma, and his brother, J'zhar, took him on an expedition, led by Sulla Trebatius, in order to try to "...bring him down slowly." The expedition also consisted of Sulla's bodyguard Umana, Valie (a thief and mage from Windhelm), Endrast (locally renowned explorer), and Yag gra-Gortwog (an Orc warrior and labor foreman). However, they were forced inside the ruin by a storm. J'darr began to go into withdrawal, and the other party members believed him responsible for the disappearance of the mage, Valie. As written in J'zhar's journal:
"I've run out of the little skooma I brought and J'darr is getting pretty bad. He's started hallucinating creatures coming out of the ice and the ruins, the others are starting to think he may be behind Valie's disappearance, but I know he would never do something like that."
However, in spite of his Skooma withdrawal, the 'hallucinations' are implied to (in part) be the Falmer who inhabit the ruins, and it is in fact the Falmer who took Valie. J'darr and his brother are apparently the only two of the group to avoid the Falmer's grasp, but it is unclear whether J'darr killed J'zhar or if the latter died from unrecorded wounds.
- When J'darr is approached, he may talk to his dead brother as if he is still alive.
- If Serana is being used as a follower while exploring the ruins, she may raise J'zhar's body to aid in the fight against J'darr.
- When approached, J'darr calls the Dragonborn "smoothskin," a nod to the Fallout series also run by Bethesda.