3. Unity Sigil Powder
Dropped from: Defeating bosses in group-only seasonal events or 4-player raid encounters (future potential).

Use: Used to Diablo 4 Items craft unique dungeon keys (sigils) that spawn co-op-only dungeons with ultra-rare loot tables.

Cooperative Upgrade Mechanics
Group content shouldn’t just drop special materials — it should also offer unique crafting tables or stations that players can only access after completing cooperative activities. Here are several ideas for that:

1. Sanctified Anvil (Clan Feature or Seasonal Hub)
Unlocked after completing a weekly co-op ritual, this anvil could:

Allow multi-player essence infusion, combining aspects or stats from multiple items.

Temporarily enhance a stat on your gear while in a party.

Offer a group crafting project: each member contributes resources, and the reward is a powerful co-op item (randomly assigned or cheap Diablo IV Items chosen).