What is voldemorts patronus?

Voldemort's patronus is not known as he was incapable of producing one. It is believed that in order to cast a patronus charm, one must have a pure and kind heart, which Voldemort certainly did not possess. Additionally, Voldemort's practice and focus were aimed towards the Dark Arts, and the patronus charm is a manifestation of a positive, protective force, which is not in line with his beliefs and intentions. Therefore, it can be inferred that Voldemort never even attempted to produce a patronus.