Riftbound chest wow
this actually dropped two keys for me and the friend who was with me, so you may be able to skip one of the other sources (unless the fifth key is used for something else in the future). from a Helgarde Supply Cache, which i think was around /way #1543 67.3, 49.0.
go down the stairs and kill Kjellrun, and on the wall to the right of where he stands is The Harrower's Key Ring, containing another key. the entrance is next to Deomen the Vortex, and the coordinates are /way #1543 66.9, 56.2. from a keyring inside a building next to a named mob.a secondary questID popped when i killed it, so i think one was the daily tracking quest for ylva and one was for looting the feeder's hand, which did not drop for me on subsequent ylva kills (but did drop for a friend on his first kill). inside a Feeder's Hand and Key, which dropped from the rare elite mob Ylva.i think the first one i got dropped from a random mob in the area when i was doing campaign quests (i'm pretty sure it was from a Maldraxxi Defector, but not positive).This is a chest inside a cage that required four total Seal Breaker Keys to open – three for the three cage locks, and one for the chest itself. I also received a possibly-related item, Nilganihmaht's Silver Ring, from a Domination Sealed Chest at /way #1543 66.0, 57.4 behind the staircase. so the construct's appearance may be dependent on whether or not that quest is active. i think i may have been doing Get to the Point - i just remember i was standing in that area for quite a while, and this is the necrolord assault quest where that makes the most sense.
i saw the announcement of its spawn 2 or 3 times while i was in the area. i was not doing anything special, but i was on a quest that required killing mobs in the area. iirc, the NPC ran across the middle part of the front of perdition hold, toward the bridge that goes to the altar of domination, around /way #1543 29.9, 61.8. it yelled the following text and continued to run as i attacked it: " You cannot have it! It is mine, stay away filthy Maw Walkers!" at ~210 ilvl, i didn't have a hard time killing it, but undergeared players might find it more difficult. I looted a Quartered Ancient Ring from a Maw Mad Construct, which was running through perdition hold while the necrolord assault was up. Comment by slumberaccording to a recent news post, the way to get this mount isn't 100% known, just that it involves combining Quartered Ancient Ring, Quartered Ancient Ring, Quartered Ancient Ring, and Quartered Ancient Ring to make Nilganihmaht's Stone Ring, and then placing it somewhere.