Northrend in general: I dont think we'll find anything there because the timelines dont add up. Places I've visited to check for clues and nothing found:ДЄll 3 Stratholme instances, Ulduar, ICC, Old Ebon Hold and the lands around it, Invincible's grave, (I think) every tomb/graveyard in stormheim (Yes also Shield's Rest), Terenas's grave, Naxxramas, Shadowmoon Burial Grounds. Random legion loot, again someone would've found it. Class hall, again someone would've found it.

Quests, if looking at all the in legion added quests, for DK's only (if other classes could do the quest it wouldnt make sense) nothing points at a quest related to this item. Legion bosses/rares, otherwise someone would've already found it. Now I have to wait a few weeks for the next quest >_> As stated by MikeTsa7 below: I'm currently trying to find зєіиђЁж‹‰ж–Їе¦й™ўй’ҐеЊ™, I doubt we will get it there because someone would've found it but it is quite interesting. I dont know if ALL raid bosses were available (i've read something about limited content) but if it was limited either Tichondrius or Gul'dan seems Obvious to drop it. Since the effect only lasts for 18 seconds and it could be used through the entirety of Azeroth. Both expensive and rather painful to test. The potions are quite expensive and with low quantity on the AH. Skaggldrynk, a friend of mine came with the idea but i also noticed someone below mentioned it. So if we have to find and talk to an NPC it's most likely a ghost or something. The phrase 'no living soul remembers' indicates that nothing alive knows it. Maybe it has something to do with the val'kyr/vry'kul. Think about it, vikings used runes as well. Why put so much effort for just a bit of artifact power.

There is definitely something with his phrases. The memory of arthas found on the broken isles. I myself dont know much about him yet but I'm about to join the discord group to find out more. It also seems to drop from bosses in ICC and Naxxramas иЇ„и®єжќҐи‡Є TimzonIncaen DROPPED BY THE SOULTAKERS WORLD BOSS

Blasphemous deathweaver