I have to say, these guys are pretty disgusting.
Armour Class 13
Hit Points 22 (3d8 + 9)
Speed 20 ft.
Damage Immunities poison
Condition Immunities poisoned
Senses darkvision 60 ft., passive Perception 8
Languages understands the languages it knew in life but can’t speak
Challenge 2 (?? XP)
Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point in stead.
Absorb Electricity. When a rust-risen is damaged by electricity, it takes no damage (as if immune). Instead, the rust-risen gains 1 temporary hit point for every 4 points of damage it would have taken. A rust-risen can only have a number of temporary hit points equal to 150% of its hit point total. These temporary hit points disappear 1 hour later. If a rust-risen takes more than double its normal hit point total in electricity damage from a single attack, the creature is destroyed.
Malfunction. The machinery embedded in rust-risen functions unpredictably in stressful situations. When a rust-risen enters combat, there is a cumulative 1% chance each round that it malfunctions. This chance resets to 0% after combat ends. When a rust-risen malfunctions it has disadvantage on attack rolls, saving throws, skill checks, and ability checks for until the end of combat.
Multiattack. The rust-risen makes two attacks, only one of which can be a bite.
Arm Drill. Melee Weapon Attack: +3 to hit, reach 5 ft., one target.
Hit: 6 (1d6 + 3) piercing damage.
Rusty metal jaw. Melee Weapon Attack: +3 to hit, reach 5 ft., one target.
Hit: 7 (1d8 + 3) piercing damage.
Miscellaneous bits of rusted machinery are fused to the body of this shambling corpse.