Legendary Resistance (3/Day). Choose to succeed failed save.

Spiny Defense. A creature that touches the sinstar or hits it with melee attack while within 5 ft. of it takes 10 (3d6) piercing damage and if the target is a Humanoid it must make DC 19 Con save or contract the thrall sickness disease (see the Thrall Sickness trait).

Thrall Sickness. A Humanoid infected with this disease manifests symptoms 1d4 days after infection which include excessive thirst increased desire for exposure to sunlight and the appearance of itchy bumps on the skin. This disease wears down the victim's psyche while slowly transforming its body. Until the disease is cured at the end of each long rest the infected creature must make a DC 19 Con save. On a failure the creature's Dex and Int scores are each reduced by 1d4. The reductions last until the infected creature finishes a long rest after the disease is cured. The infected creature dies if the disease reduces its Dex or Int score to 0. A Humanoid that dies from this disease transforms into a star thrall under the complete psychic control of the sinstar that infected it. This otherworldly disease can be removed by the greater restoration spell or similar magic.