The Grim Reaper was an AI similar to Pirates and Aliens, except it was even stronger than some Dreadnoughts at the time. You had to destroy the Grim Reaper multiple times and collect Pumpkins for the Halloween Quest. It could have dropped up to 4 at a time. The Grim Reaper is the equivalent of a Level 3 Starbase. The Grim Reaper had an icon similar to an Alien Punisher except it was orange and a lot bigger. The Grim Reaper has total 18k health, 12 Phasers, 1 Huge Cannon, and several Plasma Turrets. The Grim Reaper is able to solo a Level 3 Starbase. The Grim Reaper only spawned once per round and the server must have had at least 5 players in it for it to spawn.
Stay mobile and don't let it outsmart you, however, also try to land as many Spinal hits as you can. If you are fighting in a group, it's best to take different Ships, such as the Ridgebreaker to tank and dish out damage, a fast dread like the Slipstream, to distract the Spinals and Turrets, and a few Leviathans or Nemesises to deal a lot of extra damage. If you had a large team, a powerful Carrier could have done the job.
The Grim Reaper may be removed but is still in the game files and Rcouret can spawn it anytime he likes.