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 Diallos and complete his Questline.
First Meeting - Roundtable Hold
Diallos can be found standing near the path leading to the balcony when you set foot in the Roundtable Hold for the first time. He’ll tell you a little about himself and then remark about his missing servant, Lanya, who is a bit of a tomboy and has run off somewhere. Diallos will offer precious little to say after that, although he almost lets slip a little about his heritage.
Second Meeting - Academy Gate Town, Liurnia
Once you reach the Liurnia region after finishing up Stormveil Castle, Diallos will vanish from the Roundtable Hold as he goes to search for Lanya himself. You can find both him and Lanya’s corpse northwest of the Academy Gate Town Site of Grace and he remarks about a particular house. There’s not much else you can do for Diallos for now, and you’ll have to wait until you gain access to Volcano Manor to progress the storyline. As for poor Lanya, there’s nothing you can do to prevent her death, her passing is a requirement for progressing Diallos’ Questline.
Third Meeting - Volcano Manor
You will next find Diallos at Volcano Manor, in the Drawing Room once you’ve agreed to join the Recusants. He’ll lament his weakness and choices at having agreed to join them. Continue to return to him and exhaust his dialogue as you progress and complete the assassination contracts. Once you finish Volcano Manor by defeating the main boss there, Diallos will move from Volcano Manor.
Final Meeting - Jarburg
Jarburg is an optional village located in eastern Liurnia, to reach it, start from the Artist’s Shack Site of Grace and head south along the cliff’s edge to your left (when heading south). If you look carefully, you’ll see tombstones you can use to drop down to below, leading to Jarburg and its Site of Grace. You’ll meet a small jar sitting outside a house here and you must progress Jar-Bairn’s Quest by speaking with it and resting at the Site of Grace.
Once Jar-Bairn mentions the new Potenate, head down to the village and you’ll spot Diallos tending to a small jar in one of the houses. Speak to him and exhaust his dialogue before returning to the Site of Grace to rest once more. You may have to rest at the Site of Grace a few times to trigger the next step, or warp to a different Site of Grace before returning and you’ll notice that Jarburg has been attacked.
You’ll find Diallos, gravely injured and when you speak to him, he’ll ask you if he defended the jars. Respond that he defended well to put his mind at ease before his passing. Use the Site of Grace once more and you’ll find Jar-Bairn standing over him, exhaust the dialogue and once again use the Site of Grace. This time, you’ll find Diallos’s Mask, Hoslow’s Petal Whip, and a Numen’s Rune as Diallos tale comes to an end.