Its blood/internal organs produce and are sustained by a highly corrosive substance. Upon the creatures death all those within 5ft of it must make a dc15 dex saving throw  or take 2d8 acid damage.

All (non-magical or silvered)weapon attacks that do piercing/slashing damage and succesfully hit the creature give the weapon a -1 to its attack and damage rolls, reaching -5 destroys the weapon (rust monster rules yo).

Abberant parasite make either 2 claw attacks or 1 bite attack or 1 spit attack.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., 1 target. Hit: 10 (2d6 + 4) piercing damage. 
On being hit target must make a dc14 Con Saving throw, if not take 2d6 poison damage and become poisoned until the end of their next turn.

Claws. Melee Weapon Attack: +6 to hit, reach 5 ft., 1 target. Hit: 12 (2d8 + 4) piercing damage. 

Spit. Range Weapon Attack: +6 to hit, reach 20ft., 1 target. Hit: 12 (2d8+4) poison damage.
On being hit target must make a dc14 Con Saving throw, if not take 2d6 poison damage and become poisoned until the end of their next turn.

Infestation. Upon hitting a target with its bite, and the target failing the saving throw for the poison damage, the parasite can force the target to make another dc14 Con Saving Throw or become infested with its offsping. If the target fails they have 1d4 hours to seek a healer with knowledge of 4th level or above healing magic or suffer an awful fate.