Ebonpiercer is a Unique Amulet in Diablo IV Season 15. It drops from Andariel (Echo of Andariel).
Unique power
Blight also shoots 4 smaller piercing projectiles, each dealing [26.667 - 33.333]% of Blight's defiled area damage. (3.2.1 patch notes: 'Damage increased from 40-50% to 60-70%'.)
Affixes
+1-3 to Blight
Where it drops
Andariel, Maiden of Anguish (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