Chaos Beast

Amphibious. The chaos beast can breathe air and water.

Unstable Form. The chaos beast's body constantly shifts and oscillates. Attacks rolls against the chaos beast have disadvantage.

Immutable Form. The chaos beast is immune to any spell or effect that would alter its form.

Magic Resistance. The chaos beast has advantage against spells and other magical effects.

Multiattack. The chaos beast makes two slam attacks.

Slam. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 10 (2d6 + 3) bludgeoning damage. If the target is a creature, it must succeed on a DC 12 Constitution saving throw or become unstable. While in this state the target's speed becomes 5 feet, it has disadvantage on attack rolls and ability check, and must make a DC 12 Constitution saving throw at the end of each of its turns. On a successful save, the effect ends for that creature. On a failed save, they target's Constitution score is reduced by 1d4. If this reduces the target's Constitution score to 0, it dies and becomes a chaos beast at the start of its next turn. Otherwise the reduction lasts until the end of a short or long rest. 

Chaotic Attunement. When a creature the chaos beast can see within 30 feet makes an attack roll or saving throw, the chaos beast subtracts 1d6 from that attack roll or saving throw.