Yaerius

"In ages past, the Archangel Yaerius visited a young ascetic named Akarat and revealed to him the tenets of what was to become the religion of Zakarum. These teachings stressed the necessity of resisting all things evil, and embracing, with total devotion, the Light. The angel appointed Akarat to be the prophet of these new teachings, and bade him take the word to the people of all lands."

The Archangel Yaerius is said to be the one who seeded the foundation of the Zakarum Church. It is not certain whether he was really an Archangel or just one the demons in disguise, like Diablo as the Archangel Mirakodus which led Ureh to ruin. Not much is known about Yaerius as the only mention of him is in The Black Road.

It may be worth noting that the line that introduces Yaerius seems to project him as forcing Akarat to found the Zakarum religion.

"The Archangel Yaerius had coaxed a young ascetic named Akarat into founding a religion devoted to the Light." - The Black Road

The same book also mentions that the Demon Lord Kabraxis prevented Yaerius from bringing forth Zakarum during the Sin War. It seems Kabraxis' sole mission was to create an antithesis to the Church of the Zakarum, which he did with his own version of Akarat, another prophet named Kreghn.