In Elden Ring, you’ll find a wide range of characters that will share their plight with you, most of these will have Quests that span multiple locations. This page will detail where you can find Jerren and complete his Questline.
First Meeting - Redmane Castle
The first time you come across Jerren, it will be as the herald for the festival of war at Redmane Castle in Caelid. There’s not much you can do with him here, and you’ll have to interact with him to gain access to the Starscourge Radahn battle. After the battle is over, refresh the area and head north of the Redmane Castle Plaza Site of Grace to find Jerren sitting on a chair. Speak with him to learn that now that the war festival is over, he’ll be moving on soon.
Second Meeting - Witchbane Ruins
You’ll next need to progress through Sorceress Sellen’s Quest up until the fourth meeting, where you use her Primal Glintstone on the puppet in Liurnia. Make your way back to where Sellen was chained up in the Witchbane Ruines, Weeping Peninsula and you’ll now find Jerren. He’ll remark that he’s sure that Sellen lives on elsewhere.
Final Meeting - Raya Lucaria Academy
Once you’ve cleared Raya Lucaria Academy, return to the Raya Lucaria Grand Library Site of Grace and head outside, you’ll find two summon signs either side of the door. A gold one that will summon you to the aid of Sellen against Witch-Hunter Jerren, doing this will result in Jerren’s death and end his Questline here, preventing the rewards from that. The red sign will allow you to invade Sellen and assist Jerren, ending Sellen’s Questline and preventing her rewards, you must make a decision as you cannot complete both on a single playthrough. As for rewards, Sellen’s conclusion offers the greater rewards (you can find out more about Sellen’s rewards on her respective NPC page).
Sellen is by far the tougher adversary here, her Sorcery spells can hit a huge area so let Jerren take the focal point and attack her from behind when it’s safe to do so. Once she’s defeated, you’ll obtain Sellen’s Bell Bearing and the Witch’s Glintstone Crown. Leave the library and you’ll find Jerren standing to the right (as you exit) and he’ll hand over an Ancient Dragon Smithing Stone as way of thanks.