Prayer magic is basically when one or more sentient species put their faith into a being, (Angels mostly) then, the being(s) would use the faith and focus it with magic into holy magic and energy. Some beings other then angels also use this magic, mostly high level paladins and creatures with high standing in mythology.
This holy magic was developed by the Archangel Terra when she first arrived on the Angel Sanctuary Prometheus. She used this magic to heal herself after the long grueling trip from heaven to Prometheus. Her discovering the magic was a complete accident.
When she first landed on Prometheus, a few human knights met her. The two knights were faithful to angels, and only angels. Their faith and dedication began to fill Terra with energy and she was able to build a makeshift camp for her and the Knights. The knights helped the exhausted angel and were the first to receive knowledge of holy magic
After more knights joined and put their faith into Terra, she began to grow stronger. And she was able to aid in the defeat of the demon plague on Prometheus. She then grew more powerful for more followers started worshipping her. Now she is only strong enough to see a little bit into the future due to the small number of worshippers in The Knights of the Holy Queens.
Prayer magic has its draw backs aswell, the magic can be used only once on a follower and then the benefits start diminishing slowly. If this magic isn't done correctly then it could cause the caster to die by overdosage of energy. It also causes the user to act like a god and be there for the people that worship the caster. If not, faith may dwindle and energy gain from this magic will fail.