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Medium humanoid (dwarf), lawful evil
Enlarge (Recharges after a Short or Long Rest). For 1 minute, the duergar magically increases in size, along with anything it is wearing or carrying. While enlarged, the duergar is Large, doubles its damage dice on Strength-based weapon attacks (included in the attacks), and makes Strength checks and Strength saving throws with advantage. If the duergar lacks the room to become Large, it attains the maximum size possible in the space available.
War Pick. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 2) piercing damage, or 11 (2d8 + 2) piercing damage while enlarged.
Javelin. Melee or Ranged Weapon Attack: +4 to hit, reach 5 ft. or range 30/120 ft., one target. Hit: 5 (1d6 + 2) piercing damage, or 9 (2d6 + 2) piercing damage while enlarged.
Invisibility (Recharges after a Short or Long Rest). The duergar magically turns invisible until it attacks, casts a spell, or uses its Enlarge, or until its concentration is broken, up to 1 hour (as if concentrating on a spell). Any equipment the duergar wears or carries is invisible with it.
The tyrannical duergar, also known as gray dwarves, dwell in fantastic cities deep in the Underdark. Using ancient dwarven knowledge and myriad slaves, they work tirelessly to expand their subterranean kingdoms.
Slaves to Slavers. The duergar were once dwarves, before their greed and endless delving beneath the earth brought them into contact with the mind flayers. Held in captivity for generations by the illithids, the dwarves eventually won their independence with the aid of the evil god Laduguer. Slavery had forever changed them, however, darkening their spirits to make the duergar as evil as the tyrants they had escaped. Despite winning their freedom, duergar are dour, pessimistic, untrusting creatures, always toiling and complaining, with no memory of what it means to be happy or proud. Their craftsmanship and accomplishments endure, yet they are bereft of warmth or artistry.
Tough as Stone. Like dwarves, duergar have strong constitutions. Adding to their physical stamina is an incredible mental fortitude resulting from their time as slaves of the illithids. A duergar's mind is a fortress, able to shrug off charms, illusions, and other spells.
Born of Darkness. The Underdark is saturated with strange magical power, which the duergar absorbed over generations of imprisonment. A duergar can increase its size and strength for a short time, becoming a powerful ogre-sized warrior. If it faces a foe it can't fight, or when spying on creatures approaching its territory, it can just as easily become invisible to slip away into the darkness.
Infernal Master. Asmodeus, Lord of the Nine Hells, has been known to impersonate duergar gods in order to cultivate the evil brimming in the hearts of the gray dwarves. He offers them divine guidance and vengeance against their enemies while urging them on toward greater acts of tyranny, all the while concealing his true identity.