Lockheart not dropping from Unsynced Stone's Vigil.

[Alklios]

You're probably just unlucky; I got mine after about 20 runs. (Still a pretty lame drop rate considering how many times an actual leveling player would run the dungeon. lol)

I'd be content if I could find data or confirmation from SE that it's drop rate is not the projected 7.69% of being 1/13 weapons, but rather lower. It'd reflect the experimentation well, but loot tables are possible to be bugged.

Also, did you get it via Synced or Unsynced?