Dyson Laser

Dyson Laser is a form of an energy weapon (rarely de facto a laser, but the name sort of stick) that uses a captured star as a powersource. It is found in the form of Citadel Planets, created by an unknown species of aliens that seemed to be very much into defensive.
While it can be used to destroy warships - the power of the ray is obviously programmable - it is merely a secondary thing to its true ability. Dyson Laser is more than capable of destroying Worlds... or even entire star systems if the ray is sufficiently scattered around. Which would make it a bit more powerful version of a Lighthammer... if not for one fact.
It can do that on interstellar distance. Target an enemy planet a 100 light years away, and pull the trigger. Exactly 100 years in the future a ray of energy will reach it and sterilize it, completely wiping out all life and destroying everything ever built there. Which makes it one of the most feared weapons whose true nature is mostly hidden from the world to avoid people understanding that there can be an undetectable and unstoppable world-killing payload en route to their planet... and there is absolutely nothing they can do.