Dolmen Stone is a Unique Amulet in Diablo IV Season 15. It drops from Urivar.
Unique power
Boulder damage bonus [113-135%][x] (Patch 3.2.1: 'Damage is on entire skill, not just explosion. Damage increased from 75-90%[x] to 113-135%[x]').
Affixes
+148 All Resist
+[1-3] to Boulder (1-3)
Where it drops
World drop (any difficulty, drop rate scales with Torment tier)
Dedicated drop: Echo of Urivar (Lair Boss, Torment 1+; Boss Trophy: Judicator's Mask)
Horadric Cube: Item Transmutation (Upgrade to Unique) - Common item of the same slot + 1 Enhanced Primordial Dust + 10 Primordial Dust (random Unique of that slot)
Horadric Cube: Item Transmutation (Amalgamation) - 5x Judicator's Mask (Boss Trophy) rolls a Unique from this boss's pool; can roll Mythic quality with the 'Crafted' tag