Conquest is a real-time, multi-player space warfare game that was originally written in RATFOR for the VAX/VMS platform by Jef Poskanzer and Craig Leres in 1982. It was the early predecessor to Netrek.
Conquest can be modded via either image control files or c programming. Here we see the doomsday image replaced with the space going amoeba from "Immunity Syndrome".
This is just a gfx swap - you would have to recode the server to call it something besides "Doomsday Machine." Teams are given warning notification when their planets are attacked.