Addle Heart

Sunlight's Kiss. If this creature is to be in any square that has sunlight, direct or not, on that square, this creature would be instantly defeated, without rewarding anything but a "possible" item drop.

Moonlight's Kiss.  If this creature is to be in any square that has moonlight, direct or not, on that square, this creature would receive an extra +5 to Armour Class, making it harder to hit.

Unresting Nature. Chooses any two of these attacks per round.

Corporeal Clap. Melee Weapon Attack: +5 to hit, reach 30 ft., 1 target. Hit: 14 (2d10 + 5) Necrotic damage. 

Essence Drain. Ranged Weapon Attack: guaranteed to hit,  reach 100 ft., 3 targets. Hit: 24 (3d12 + 4) Necrotic damage, healing Addle Heart for that much damage.

Ethereal Lance. Ranged Weapon Attack: +10 to hit, range 45/90 ft., 1 target. Hit: 30 (3d20 + 10) Necrotic damage.

Rending Flesh. Ranged Weapon Attack: +5 to hit, range 45/90 ft., 3 targets. Hit: 45 (2d100 + 20) Necrotic damage.

Enduring Pain. If this creature is to be hit for critical damage, instead of being hit, Addle Heart is to teleport away immediately, taking no damage that round. This happens every time a critical hit is to be dealt to Addle Heart, and can only be activated once every two rounds.

If the monster is be in direct contact with moonlight, then this creature can cast Archaic Memory. The Addle Heart can use this action only once per moon cycle.

Archaic Memory (Costs 1 Action). As the Addle Heart reminisces about their past existences, all damage the Addle Heart does to any target is granted an extra 15 Lightning damage. This will last for three rounds. At the end of the Addle Hearts third round after this action is spent, the Addle Heart is to explode, causing 5d8 piercing damage to all within 60 ft. The players do not receive experience this way.