Draugr

Draugr Forms. The Draugr has multiple forms. The first form is the mycus form. While at this form the Draugr has the Invinsible  condition, can take only it's Infect Undead action and it's speed is reduced by 20ft. The second form is the body form. When the host dies the Draugr is transforming the host and itself into the body form. While at the body form the Draugr is Large instead of Tiny. 

Sickness of the Dead. Draugr is a mycus that infects the corporeal undead, like zombies or wights. When it infects an undead the undead suffers the following effects while Bloodied: It takes 1d6 poison damage that bypasses Resistance and Immunity at the start of each of it's turns and has Disadvantage at all attack rolls.The rest of the stat block features the Draugr's body form.

Multiattack. The Draugr can make two Acid Touch attacks.

Acid Touch. Melee Weapon Attack: +7 to hit, reach 5 ft. Hit: 22 (4d8 + 4) Acid damage.

Infect Undead. The Draugr infects an Undead within 15ft. of it. The Draugr disapears and enters the undead and the undead becames the new host. While inside a host the Draugr in Invisible, can't be attacked, has Immunity to all damage and can't take any actions, reactions or bonus action.

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