True invisibility. As they are undead very similar to spirits, they are completely invisible to any living creature even with other spell like See Invisibility. Only a Vengeful Spirt can see and fight them. They can only attack living creature and since they are invisible and undetectable, their attack count always as suprise attack.

Multiattack. This creature makes two Atrophy attack.

Atrophy. Melee Weapon Attack: +10 to hit, one creature. Hit: Shadowy Construct will hit his target doing 20 (4d8+4) slashing damage to the target and it also fall prone.