The Vault Dweller must first speak to Seth, who says that Jarvis, his brother, has been poisoned, and that they can speak to Razlo for more information. Razlo is located in the building to the South. He will then tell the Vault Dweller that he can create an antidote to the venom, but he requires the venom sack from a radscorpion, which is inside their tail.
Once the Vault Dweller has obtained a radscorpion's tail, bring it back to Razlo, who will create an antidote and give the Vault Dweller a free sample for personal use.
Only then can the Vault Dweller open the curtain to the back of Razlo's hut where Jarvis is laying on a mattress. Using the antidote on Jarvis finishes the quest.
Regardless of the Vault Dweller's Intelligence, they can still use antidote on Jarvis after obtaining it. Vault Dweller with an Intelligence of 4 or less will not be able to learn about this quest from Seth, instead visiting Jarvis in the back of Razlo's hut and trying to talk to him. He will then give some floating dialogue telling the Vault Dweller about his ailment, and that he/she should see Seth if he or she wants help removing the radscorpion threat.
Scorpion tails will be in the inventory of all radscorpions on death; however, they are quite heavy, weighing 20 pounds per piece.
If the Vault Dweller speaks to Aradesh and agrees to help his people by destroying the radscorpions, they will begin the quest Stop the Radscorpions. Part of the quest involves eliminating radscorpions inside their caves. There are a total of nine radscorpions inside the caves.
Radscorpions will spawn around the entrance to Vault 15 the first time the player's character visits.
Radscorpions may be randomly encountered anywhere on the world map.
The Vault Dweller can continue to bring back more scorpion tails, and Razlo will continue to make antidotes out of them. However, it does expend a few hours of in-game time for each additional antidote they ask him to make.
While the Vault Dweller does learn about Jarvis' condition from Seth, it is not necessary to speak with him to be able to cure Jarvis.