Frostburn is a Unique Gloves in Diablo IV Season 15 for All classes. It comes from General Unique Pool - every Lair Boss (Initiate, Greater, Exalted, Belial) can drop it in addition to their dedicated table (one guide's Boss Loot Table Cheat Sheet).
Unique power
Lucky Hit: Up to a 50% chance to Freeze enemies for 1 second.You deal [40 - 50]%[x] increased damage to Frozen enemies.
Affixes
Primary Core Stat
Lucky Hit Chance
Where it drops
General Unique Pool - every Lair Boss (Initiate, Greater, Exalted, Belial) can drop it in addition to their dedicated table (one guide's Boss Loot Table Cheat Sheet)