maybe not a planet but a mini blackhole tugged by a small ship at high velocity to crash into your sun and turn it into a proper black-hole with accretion disc

maybe not a planet but a mini blackhole tugged by a small ship at high velocity to crash into your sun and turn it into a proper black-hole with accretion disc and the works would do some serious damage to you as a species. personally I'd go for something like a nanobot gray goo thing that infects & turns everything it touches in a given gravity well. then send over a bunch of ships to wipe out remaining stragglers.