While Diallos is not a vendor or a summon, his quest (combined with Jar-Bairn’s) is one of the narrative highlights of Elden Ring, and has some decent rewards, including 
Diallos’s Mask and 
Hoslow’s Petal Whip.
How to Complete Diallos’s and Jarburg’s Quest
Speak to Diallos as the Roundtable Hold, where he will tell you about his missing servant, Lanya, and ask you to help find her.
WARNING: If you slay Rykard before completing this step, Diallos’s quest will stop here and be impossible to complete.
Find Diallos in Northern Liurnia, near the Academy Gate Town Site of Grace. He will be atop a roof amongst the sunken ruins, and can be found by his lamenting over Lanya’s corpse.
Diallos will return to the Roundtable Hold. Return and exhaust his dialog, telling you what he knows about the Recusants.
Diallos will go to the Volcano Manor, where he will remain until you accept the second assassination letter as a part of Tanith/Volcano Manor’s quest. After this, exhaust his dialog.
Diallos will move to Jarburg in order to protect it. Progress Jar-Bairn’s questline until he talks about the new Potentiate who’s arrived (which will only happen after defeating Radahn). Rest, then speak to Diallos, then Jar-Bairn, exhausting their dialogue to advance the quest.
After mentioning the new Potentiate, reload Jarburg. It will have been attacked, leaving Diallos wounded. Exhaust his and Jar-Bairn’s dialogue, telling Diallos that he either succeeded or failed to defend the jars.
Either way, he will die, leaving behind Diallos’s Mask, Hoslow’s Petal Whip, and a 
Numen’s Rune. Exhausting Jar-Bairn’s dialogue and reloading the area will lead to Jar-bairn scavenging Diallos’s remains to take up his legacy.
