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.
J'darr can be heard shouting at his brother when first entering Alftand, and later talking to his brother when he sees the player:
"Where is it? I know you are trying to keep it for your self, J'zhar... You always try to keep it for yourself! No! There's got to be more Skooma... Shut up! Shut up! Don't lie to me, J'zhar! You hid it! You always try to hide it from me!
[seeing player] What? Who is this, brother? Another of the smoothskins looking for food? But this one wasn't trapped with us..."
- J'darr talks to his dead brother as if he is still alive, so this is likely a hallucination brought on by Skooma withdrawal.
- 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.
- When Wujeeta asks to be cleansed of Skooma in her system, she asks for a healing potion to wash the "poison" from her body. However, there are a few healing potions in J'darr's inventory, as well as one in the knapsack by his brother's body. J'darr uses the potions in combat, but they do not have the same effect on him as they do on Wujeeta. This is likely because J'darr is already in withdrawal.
- If sneaking and undetected, one can listen to J'darr's "idle" quotes, which are the same as bandits' quotes.