i'll say what i see in my theory: as darkterror left his homeland and arrived to the earth in search of the problem, he casually took a tail in the face of dark spirit -- the horror and awe itself. but meanwhile darkterror is exploring new planet, this essence of fear gets in goddess Nyctasha's labyrinth of dreams where it will get material body, a shell.
it's tendrils absorbs goddess' power, her inky blood... and the birth of Bane brought in the minds of every mortal and god the wave of nightmares and awe.
as he finally leaves the mind of unconscious goddess he wants to find alien (or it is more correct to say - his compatriot? xd) from Claszureme, and will find. but for Darkterror such ally in the face of Bane elemental will very helpful, so he decides not to banish him but accept.
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i'm really hate the fact Bane's canon lore doesn't say WHYYYY WHY DID HE BORN. why he has this form? why did Nyctasha had a nightmares and her mind born such creature? these innuendo are accompanied with his such similarity with FVoid and the mount of parallels between them. FVoid arcana's response to ally Bane "together we'll make nightmares come true." was the last straw for me tbh. especially if we will pay attention to the fact FVoid is spiteful to many other even ally heroes...