Act II: Rites of the Deep
You'll need to find at least three shards of Willbreaker to reforge it. Acquire at least 3 "Willbreaker" quests from the Shaping Slab vendor.
Choosing to find additional shards will make it slightly easier to reforge Willbreaker.
In order to reforge Willbreaker, you'll need to find its shards spread across the gloam of the Dreadnaught.