

The game is notable for its hidden play mechanics, from which it derives much of its playability and tension. A Terminator piece has few action points a Genestealer piece has many action points. The pieces are moved by the players through a system of "action points", where each piece has a certain number specified for it. Terminator pieces excel at ranged combat, but are weak in close combat the Genestealer pieces excel at close combat, but cannot perform ranged combat at all.The pieces of the two players move at different rates (Terminators move slowly, Genestealers rapidly).For example, the Terminator player may have the objective of destroying a specified area of the board or a specified Genestealer piece or some other objective while the Genestealer player may have the objective of destroying a specified Terminator piece or all of the Terminator pieces or some other objective. The players have different objectives to fulfill during a "mission" (the in-game term for a particular scenario).It is an asymmetric game in the following respects: One player controls the Space Marine Terminators, and the other player controls the Tyranid Genestealers. The game is set on a modular board made up of various board sections which represent corridors and rooms and which can be freely arranged and locked together like a jigsaw puzzle to represent the interior of a space hulk. Besides a space hulk, similar skirmishes between Space Marine Terminators and Genestealers can also be set in other environments. Space Marine Terminators were originally only used in Space Hulk-type scenarios, but rules were added in White Dwarf and subsequent releases of Warhammer 40,000 and Epic for their deployment in conventional battles in limited numbers as elite troops. Space Marine Terminators are described in game lore as veterans who have earned the right to don special armor.

Since the 1990s, subsequent games have absorbed them as part of the overall Tyranid army, where they serve as the shock troops. Genestealers were described in the first edition of Warhammer 40,000, but they were very different from their Space Hulk incarnation, which was more influenced by the xenomorphs depicted in the Alien franchise. Space hulks may house many threats: genestealers human followers of the dark gods of Chaos daemons and Orks, who use space hulks as their standard method of interstellar travel. Space hulks are often agglomerations of multiple ships, fused together by the magical influence of the Warp, a kind of hyperspace. In Warhammer 40,000, a "space hulk" is any massive derelict space ship.
