Cassia's Grace is a Unique Bow in Diablo IV Season 15 for Rogue. It drops from Urivar.
Unique power
Your Arrow Storms last 2 seconds shorter, deal [60 - 90]% increased damage, and, when created, have a 50% chance to create an additional Arrow Storm on a nearby target.
Affixes
Ranks to Arrow Storm (+1-3)
Where it drops
Urivar (Fields of Judgement, Nahantu; requires Vessel of Hatred) - Lair Boss, 1x Lair Key (boosted/targeted drop)
Belial, Lord of Lies (Palace of the Deceiver, Kehjistan) - mimics any chosen boss loot table, guaranteed Ancestral Unique, highest Mythic chance