Tenshi no Himegoto

Hideyuki Inori is an exorcist and a member of a secret Catholic order fighting supernatural evil worldwide, but apparently even Holy Ops from the Vatican need a vacation sometimes, because he suddenly asks for a leave from service and returns to his native Japan.
His true motives, however, are terribly serious: his dearest childhood friends, the two daughters of his old teacher, have fallen under demonic influence, a power so terrible that maybe only the death of the girls themselves can banish it.
Hideyuki is unwilling to report this situation to his superiors, but what can he do? And what are the real nature and objectives of this mysterious, demonic enemy?