Legion follows a standard horror thriller format: a group of strangers with various and initially hidden motives are trapped together in an isolated location surrounded by hordes of murderous bad guys. The twist in this case is that the bad guys are a plague unleashed on the Earth by a dissatisfied God and they have a special mission to carry out with one of the members of the group.
Much of this film is by the numbers isolation horror and the characters are exactly what you would expect. There is little or no development to the characters and as they get killed off it feels like the writer is fulfilling his quota for the film rather than advancing the plot or characters in any way. It definitely feels like they started with the end of the film in mind and worked back from there so that everything fits neatly to their framework without thinking about anything else.
There is the usual very poor decision making from both sides in the fight and jarring periods of sitting around talking and introspection when realistically they should be fighting for their lives or at least securing the area or running like hell.
The God and Angel stuff is given an interesting modern twist reminiscent of both Dogma and Terminator. Angels fight with automatic weapons as well as more traditional stuff (a morningstar was a nice touch) and some of their appearance and equipment seems more technological than biblical. This gives the film a nice difference in appearance from most other films that feature angels. However, most of the religious stuff is way overdone and delivered as exposition which drags after a while.
After all, although there are some interesting concepts and departures from the usual “bad” guys in this film essentially they have just substituted biblical end of days for whatever kind of virus outbreak/demonic possession/toxic spill/radiation blast etc that causes hordes shambling evil to be unleashed upon the world in more conventional horror films.
Whilst mostly being a “straight” survival horror film, there are some great funny parts here and these are usually stolen by individual evil characters. Some are just brilliantly quirky while others are ridiculously over the top and some are inept but those that get any decent amount of screen time are generally likely to raise a smile. The same cannot be said for the Angel characters who (perhaps unsurprisingly) are relentlessly serious throughout and pretty boring even when they get stuck in to the fighting.