Tremulous is a free, open source game that blends a team based FPS with elements of an RTS.
The achievements in the game allow the aliens to evolve and leave the initial classes: Granger and Dretch.
Each annihilation gives humans a number of credits that can be exchanged for weapons and equipment.
The overall objective behind Tremulous is to eliminate the opposing team.
This is achieved by not only killing the opposing players but also removing their ability to respawn by destroying their spawn structures.