Bloodletter

Feel No Pain. The bloodletter will fight to the death once summoned. They feel nothing, even if they lose all limbs. Only death will stop them.

Magic Resistance. The bloodletter has advantage on saving throws against spells and other magical effects.

Blade of Bone and Flesh. The bloodletter cannot be disarmed as their hellblade s part of their body.

Multiattack: The bloodletter makes two melee attacks with its Hellblade. 

Hellblade. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) piercing damage. This weapon is counts as magical for the purpose of overcoming resistance and immunity to non-magical attacks and damage.

Invisibility. The bloodletter magically turns invisible until it attacks or uses Scare, or until its concentration ends (as if concentrating on a spell). Any equipment the bloodletter wears or carries is invisible with it.