Protean Cell is a 
Order Armor Shield
Protean Cell Ground slam drains shield over time for +40% shock dmg.
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How to Unlock the Legendary Protean Cell Shield
Legendary Loot
Shields
Overview
How to Unlock
Unlock Location:
- Progress through the story until reaching "One Fell Swoop" main quest.
- Go to Carcadia Burn and find the Crawlcroft Order Bunker
- Progress to the end to find Fractis, the boss who will drop Protean Cell.
Farm Method:
- Defeat Fractis.
- After use the Encore Machine nearby to reset the fight.
- Repeatedly defeat Fractis until Protean Cell drops.
Amon
Crucible
Loveless
Malignant Construct
Vex
Dead Ringer
Harlowe