The Undercrown is a Unique Helm in Diablo IV Season 15. It comes from Duriel, King of Maggots (Lair Boss) per one guide; Random world drop / any Lair Boss (one guide: 'drops at any Difficulty Tier, increased drop rate per Torment tier'); 3.2.1: all Uniques drop from level 1 in any Difficulty; targetable via Horadric Cube reroll.
Unique power
Your maximum number of Skeleton Warriors and Skeleton Mages is increased by 4 and your Summon damage is increased by [15.0 - 25.0]%. Commanding Skeleton Warriors also causes Skeleton Mages to focus the same target for 5 seconds.
Affixes
+1-2 to Minion Skills
Where it drops
Duriel, King of Maggots (Lair Boss) per one guide
Random world drop / any Lair Boss (one guide: 'drops at any Difficulty Tier, increased drop rate per Torment tier'); 3.2.1: all Uniques drop from level 1 in any Difficulty; targetable via Horadric Cube reroll