It's gregorian chant, it is still sung nowadays in some monasteries and catholic churches

Do you maybe know the name of it by any chance?

It's 'O vrigo splendens', the entire track is one song, with a very long interlude. At 5:17, it's the line 'ne infernorum...'

@@alcoolamus4208 Yes I do know that, but I was trying to find out where did this sample of this particular Greek-Orthodox chant originated? Or did the composer made it for the game?

Gregorian Chants, the same kind from the Halo theme