To bring the age of darkness to the world. Femto as the wings of Darkness, got reincarnated as Neo Griffith, a savior. According to the prophecy of the Falcon, which Schierke knows about:
When the Sun has died five times, a red lake will appear at the West of the city with a name both new and ancient, and it will be the sign that the fifth angel, the Falcon of Darkness, is born. The Angel shall be both master of the sinful black sheep and king of the blind white ones. He is the one who shall bring an age of darkness upon the world."
This refers to Griffith's rebirth as Femto, and after his incarnation in Albion, the gathering of apostles for his army. The "age of darkness" may refer to his future reign.
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As explained by the prophecy, Griffith is the master of the sinful Black sheep= Apostles through his true form Femto, but he's also the king of the blind white ones aka humans. He will bring an age of darkness which could be what his reign will be like.As to what that entails, it could have to do with the appearance of magical creatures all over the world after the merging of layers. Some of them aren't bad, but many of them can be destructive like the trolls and the sea god for example. What he intends to do is gain the trust of humans by having the apostles fight the magical creatures, and as we've seen many humans have taken refuge in Falconia, where Apostles and humans coexist, although the apostles have their own quarters. It is not clear if Griffith is really trying to help humanity,or if at one point he will sacrifice it for some obscure reason. I can't help but see some similarities between Griffith and Gaiseric's reign. It is implied that Gaiseric's reign ended with a massive sacrifice, and possibly the wise man ascending as Void. I think it is also mentioned that his reign was decadent, that the "angels" were sent to punish him.And there's the fact that Falconia is literally made of the remnants of the old capital from Midland.
So all we have as a clue is that Griffith's reign will be an age of darkness and when he ascended the other god hands mention that if I'm not mistaken.
So from Slan's words, evil shall overcome the sacred, well we already know the God hand are evil and all over the land we've seen some pretty bad things like what was done by Mozgus for example.Not just that but Griffith is posing as the Falcon of light, a savior when in reality he's a demon. Illusion shall overcome existence, we've seen Griffith perform "miracles", like bringing the souls of the dead. I always felt that those were illusions, and not the real souls of the dead.Hate shall overcome love, fear shall overcome hope etc, this is something that is pretty obvious. Right now humans are losing hope as they are being attacked by all kinds of magical creatures. The dead shall overcome the living. This part could also be related to the one with the souls, but the apostles are also dead in a way, and they were reborn thanks to the behelit.And darkness shall overcome the light, which means it will be an age of darkness basically.
Overall I think Griffith's goal will end up meeting that of the God Hand, and IoE of course if it's still canon.He was allowed by IoE to do as he wishes, but in the end it will meet IoE's plan, who manipulates the possible routes through causality.