The Black Templars.

(Bot request for @affkf and a few others. Sorry, I've lost track of exactly who else asked for it. M1--Generic starting intro. The Templars have been called to crusade. Have fun. M2--A Thunderhawk containing a Squad of Black Templars is shot down as they attempt to land near a Hive city. It is implied the city itself has been infested by a genestealer cult but it's left vague enough that you can insert whatever type of 'cult' you want.

Warning for religious themes, zealotry, Xenophobia, violence, and general Warhammer 40k themes)