Diallos at the Roundtable Hold
When you initially visit the Roundtable Hold, you will have a run-in with Diallos. If you talk to him, he will explain what has happened to his companion, Lanya, who has gone missing. While you are on your travels, he will ask you to keep an eye out for her.
Diallos at Liurnia of the Lakes
You’ll run into Diallos once more close to Academy Gate Town as you make your way through Liurnia of the Lakes on your way to the Academy of Raya Lucaria. When you leave the Site of Grace and head north, you will find Diallos lamenting over the body of Lanya as he stands over her grave. Have a conversation with him, then head to the Roundtable Hold, where he will appear once more. Repeating his lines of conversation here will eventually result in him moving into Volcano Manor.
Diallos At Volcano Manor
You can find him in the drawing room of Volcano Manor, which is also where you will pick up the letters necessary for the Tanith Questline assassinations. He will be positioned directly to Bernhal’s right.
If you talk to him after you’ve completed your first assassination, he’ll leave Volcano Manor and come back later looking defeated. When you have finished all of the quests at Volcano Manor, Diallos and the other residents will leave at the same time.
Diallos at Jarburg
Jarburg, which is located to the south of the Carian Study Hall, is the location where you will find Diallos next. You can find him muttering into a jar while he’s holed up in one of the small huts. Since he has left Volcano Manor, he claims that he is now pursuing his own course of action.
After leaving the area and allowing some time to pass (around three to four days in-game), you will discover that Jarburg was attacked by a poacher in the interim, and that he killed all of the jar people. After making an unsuccessful attempt to protect the jars, Diallos will be found lying on the ground, mortally wounded. He will enquire as to whether or not he was successful in defending the town; regardless of whether or not you choose to tell the truth, this will not affect the progression of his quest.
After the area is loaded again, Jar-Bairn will be standing close to Diallos, and they will say that they aspire to be a powerful warrior just like Diallos. You will be able to recover Hoslow’s Petal Whip, Diallos’ Mask, and a Numen’s Rune from Diallos’ body if you perform one last reload of the area.