These range from having your one flamethrower marine burn specific rooms, to picking up bombs and taking them to specific areas, to simply digging in and surviving for a specified amount of time. Once you've secured an entry area, mostly by putting marines at the end of any passages leading into it, you can send the rest of your squad to complete your mission. They literally pour out of these access points, and can overwhelm your forces in a matter of seconds, if you haven't set up to contain them. These presumably are coming from other areas of the ship, but in practical terms, it means that you will have a few key zones that your enemies are coming from. Genestealers "spawn" from locked doors marked in red on your map. The maps throughout the game are made up entirely of long passages and 90-degree angles, so you have to figure out how to best place your marines to lay fire down key hallways and access points, or seal doors to keep the aliens out. On the other hand, though marines have some basic hand-to-hand capabilities, they will usually die if any alien gets too close. One marine can splatter Stealer after Stealer, as long as he has some distance to work with. This is the basic point of the game, and what the entire strategy is based on. A few missions will pit you against other enemy types, like "hybrids" that look like two-armed Genestealers with guns, but most of the time its your long-range Marines against the short-range Stealers. They do have numbers in their favor, and can swarm from multiple directions, or send relentless streams of foes down a hallway after you. They have multiple arms, making them vicious in melee combat, but sport no ranged attacks. You fight a group of purple, shark-looking aliens called Genestealers. They use pretty common weapons, so controlling them should be familiar to you.
You play as fanatically religious marine-monks who pilot walking suits of armor called Terminators. Vengeance centers around two warring factions. The game doesn't appear to be using any pen-and-paper or RPG rules anyway, so you can consider this game "inspired by," but not a replication of, the tabletop experience. I must admit that I am not familiar with them, so I'm approaching this review from a strictly video game standpoint.
Space Hulk: Vengeance is based off of the Warhammer 40K tabletop games created by Games Workshop.