The Nicoll-Dyson Laser was proposed by James Nicoll in 2005, though it's the bastard offspring of 'Doc' Smith's Sunbeam (~1941). Basically it's a giant death ray that uses a solar system for it's construction and a star as it's power source.
It consists of a Dyson swarm (a vast number of solar energy harvesting satellites orbiting in a dense formation around a star) with the collection elements combined with phased-array laser emitters. Together they can produce a coherent laser beam potentially in the petawatt output range (or even higher) and capable of interstellar ranges. Such a network might be a useful defense against attack (perhaps by relativistic kinetic projectiles), or a weapon intended to destroy other planets (slowly, given the light-speed propagation of the beam).
Perhaps a long dead civilisation sent out self-replicating probes to 'colonise' other stellar systems and build such devices to eliminate other intelligent life that might compete with them?
Time, time,time, see what’s become of me. While I looked around for my possibilities... My AITAS files.