If the Protagonist is on the Law route and visits the room Mastema had been located at in Sector Grus, he will be greeted by an Archangel who informs him of the ritual he and some other angels are planning to do, which will create another angel to help in the upcoming battles, and asks that he come to Sector Horologium to bear witness to it. There, each angel of the Divine race is gathered and begins to sing a chant that results in a mass fusion of all the angels, which results in the creation of a Seraph. Once manifested, the Seraph praises the Protagonist for his actions, and believing he has earned the right to become a saint, offers to give him the Megido Fire, though it first asks that he complete the EX Mission "Evil's Lingering Shadow," which involves slaying Mara, who has taken up residence in the room Maya had inhabited in Sector Grus. Once the Protagonist completes the mission, the Seraph appears to reward him with the Megido Fire and the status of a saint and joins his party.
Alternatively, on the Chaos route, Mara gives the Protagonist the EX Mission "Mara's Desire," where he asks that he stop the ritual the angels are doing in Horologium. Regardless of the Protagonist's actions, he is unable to stop the angels from fusing into Seraph, though he is still able to defeat it and complete the mission.
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Reason: Missing information on seraph's appearance as a boss within purgatorium
In the New Game+Challenge Quest "Four Wings, Four Heads," Flynn is asked to slay four demons, those being Chimera, Morax, Gryphon, and Tokisada, to use them as sacrifices to create a new angel. Each time Flynn slays one of the demons, a Principality goes and collects their heads. Once all the demons have been defeated, Aniel appears to begin the ritual to sacrifice the heads, which represent the heads of a Lion, Bull, Eagle, and Man respectively, creating a Seraph, who leaves with Aniel shortly after.