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Dark Justiciar Helm
Dark Justiciar Helm
Uncommon
Swathed in Shadow: While Obscured by shadow, the wearer gains +1 to Saving Throws when attacked.
ConstitutionSaving Throws +1
Stamped with Shar's own sigil, this helm was worn by the Nightsinger's own crusaders. The darkness within seems deeper than night, somehow.
Medium Armour
Location - Dark Justiciar Helm
Act 1
Located in 
Grymforge. Found on a skeleton.
Act 2
You can find two helmets in Shadowfell. Both of them are lying on the ground next to a skeleton (1, 2). I recommend following the step-by-step guide of Gauntlet of Shar and Shadowfell to have an easier progression through the dungeon.
Discussion
Underdark: In a skeleton above the lava, next to the handles at X -603; Y 323.
Gale can absorb the Magic from this Item. If Gale consumes it, it vanishes forever. That is irreversible and unavoidable.
(There is better food for Gale: Ring of Color Spray, Frayed Drow Hood, Sunwalkers Gift, Everseeing Eye Necklace, Exterminators Axe; Hunting Shortbow, Kormira's Locket, Spiderstep Boots.)