Deathwatch by Steve Parker

When they first came together, Talon Squad were a disparate group of Space Marines from various Chapters. Under the auspices of the Deathwatch, trained in an ironclad Watch Fortress, they become a kill team. Alien hunters and experts in xenos extermination, their first mission is a deadly one. Of all the enemies the Deathwatch face, the genestealers are among the fiercest and most difficult. Under the direction of a mysterious Inquisitor Lord known only as Sigma, they must cleanse this planet or die in the attempt.