Complex sentence: In the anime adaptation, Eibon allies with Arachne 800 years in the past to cure his lover from a fatal illness and, after failing, transforms himself into a demon tool used to unlock Brew as penance.
Split into simpler sentences: In the anime adaptation, Eibon allied with Arachne 800 years in the past to cure his lover from a fatal illness. But Eibon failed and resurfaced years later to transforms himself into a demon tool used to unlock Brew as penance.