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Tremulous is a freeware first person action game based on the ioquake3 game engine. It originated as a mod for Quake 3, but eventually went standalone. The game is presumably set in the future, where humans face off against spider-like aliens.
Aliens evolve by using points earned from killing humans. All aliens regenerate health automatically.
DretchStage 1
Cost: 0
Starting class. Can bite humans simply by coming into contact with them. Can also walk on walls and ceilings.
Granger
Stage 1
Cost: 0
Builder class. Has no attacking abilities.
Stage 2
Cost: 0
Upgraded Granger. Can now slash, lob projectiles and walk on walls and ceilings.
Stage 1
Cost: 1
A stronger attacking class than the Dretch. Has a basic slash attack, and can restrain a human's movement when contact is made. Can also wallwalk.
Advanced BasiliskStage 2
Cost: 2
Effectively the same as a standard Basilisk, but has the ability to spray poisonous gas. Humans wearing battlesuits are immune to it, however.
MarauderStage 1
Cost: 2
Can bite and has the ability to wall jump, making it very agile and difficult to hit.
Advanced MarauderStage 2
Cost: 3
Effectively the same as a standard Marauder, but has a chain lightning attack which can affect up to 3 humans if within range.
DragoonStage 1
Cost: 3
Has a biting attack and a pounce ability, which can be charged and damages anything in its path once released.
Advanced DragoonStage 3
Cost: 4
Effectively the same as a standard Dragoon, but has the ability to shoot long ranged spiked barbs. 3 can be held in reserve and regenerate over time.
TyrantStage 3
Cost: 5
Has a primary slash attack, and a secondary trampling ability that can be charged up and enables the player to run at high speed for a short while. It also has a healing aura which doubles regeneration rates of lower class aliens.