The Andromedans are intergalactic invaders from the M31 (Andromeda) Galaxy, and spent 200 years on the journey to our galaxy. They have a base in the Lesser Magellanic Cloud. Their ships were first identified in 2566 (Y166); they launched a full-scale invasion in 2588 (Y188).
Absolutely nothing is known about the Andromedans as beings. Apparently, no one has ever seen an Andromedan and lived. Andromedan boarding parties are composed of robotic combat systems, leading some to theorize that the Andromedans are themselves robots or computer software.
Andromedan ships evaded contact at first, but after Y188 they attacked without provocation.
The ships are equipped for long-range travel, but do not appear to be intended to return to Andromeda.
With the conlcusion of the General War the Andromedans struck the galaxy in full force. With the ISC so disperesed they were an easy target with the Andromedans detroying two-thirds of their peacekeeping fleet. The Peace of the Galaxy was quickly shattered and War returned to a burning galaxy.
Andromedan ships use phasers (at least, something that looks like a phaser), but the rest of their technology is radically different. They use tractor-repulsor beams as heavy weapons, power absorbers for defense, and the displacement device for a variety of operations.
No official or direct contact was ever initiated with the Andromedans
The Andromedans are seen in various products, including SFB Module C2, Federation Commander: War & Peace, GURPS Prime Directive, PD20 Final Frontier (future release), F&E Andro War (future release), Starline 2400
Star Fleet Battle Force preview in Captain's Log # 35