WARNING! IF YOU HAVE NOT READ ALONE OR ESCAPE YET, DO NOT READ THIS. IT CONTAINS MAJOR SPOILERS!! 

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I think this song suits Brethren because 

First: He is a beast. Like, he's Darkness himself, and he's a really creepy, strong guy. Like, he's THE DARK LORD. Of course he's a beast! Shintaro and Cami both know this. And he's coming to play with EARRRTH-

I'm strong enough to enter the fray

You were whispering my name

Now I'm finally taking aim 

And the predator's becoming the prey

That part probably makes the most sense, at least to me. 

I'm strong enough to enter the fray (He's gained enough power to escape the Abyss, with the help of the key, and enter Earth to play.) 

You were whispering my name (Lots of people, especially those with magic, know about The Dark Lord, though most of them think he's just a myth. The ones who worked under Thorn or Moon knew otherwise, because that's what they've been trying to do from the beginning - bring their master back.)

Now I'm finally taking aim (He's preparing for the final battle that he knows he's going to have with his son and his enemies. He's ready to aim his final move at anyone who dares try to overthrow him.)

And the predator's becoming the prey (Referring to Shintaro! Shintaro is a predator to the Darks that already lived on Earth, because they aren't as powerful as The Dark Lord, and Shintaro can easily defeat them. But Shintaro becomes small and insignificant to Brethren, because Brethren is literally Dark Magic that could defeat Shintaro any day. At least, that's what Brethren thinks.) 

Any of your kind can become one of mine (Meaning that any human being is capable of being transformed into a Dark - given Dark Magic. Take Kai, for example. He was a normal kid, until Thorn gave him a piece of his own magic.)

Now the beast has come to play (Brethren has now entered Earth and is playing with anything he can get his hands on - including Kai.)

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That was Brethren's theme song! Thanks for checking this out!! :)