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Hardened Scales is an effect used by Tequatl the Sunless; The amount of stacks does reduce damage taken, but at 20, Tequatl becomes invulnerable and summons a Bone Wall in front of him. To remove the stacks, use the Hylek Turret's Scale Penetration skill.“
Tequatl's scales harden over time. At 20 stacks he becomes invulnerable.
— In-game description