Fusion beasts do not appear in the enemy beastiary.
Ok first of all this boss will be hard to deal with its high defense and high HP so with like the medium fusion spirits transform with its opposite fusion either med or high. You may want to pack some Thera Roots as will because high level fusion monsters can take away quite a bit of damage. Just nail him with your special skills and he will go down in a few turns.
Upon its defeat, the Czernobog fusion is unlocked.
Czernobog (litteraly "the black god") is the deity of night and moon in the Slavic mythology, based in Eastern Europe. He's often considered as a god of death and darkness, because of his role in the movie Fantasia or according to Christian sources. However, since the 20th century, he's actually seen as a "neutral" god, neither good or bad. But, given the lack of reliable sources and the prominence of oral traditions in the Slavic culture, its role is quite open to interpretation.
Logically, his counterpart would be Belobog, the "bright god" of day and sun, but this god was never mentionned in the culture. His actual counterpart is most likely Svetovid, god of fertility and abundance, but even that is merely speculation.
t's the fusion used by Ben Hyuga in the flashback sequence.
In the Shin Megami Tensei franchise, he looks like a mushroom holding a sword. Belobog made a few appearances,