That is Ganishka, ruler of the Kushan Empire and a rogue Apostle of the God Hand.
He looks like this on his human form:
But when he gets angry you can see that's there is something unnatural about him:
He was probably the oldest and most powerful Apostle, but he defied the God Hand, and quickly realized how weak he was compared to then.
He turned into that thing (a mindless beast of the apocalypse) when desperate to achieve ultimate power he got inside a demonic machine his sorcerers made that was meant to turn regular human babies still in their mothers womb into artificial, weaker Apostles for his demon army.
Guts killed hundreds of titan sized monsters that formed out of the Beast of the Apocalypse body.
When he was finally defeated in a joint effort by Griffith, Guts, Zod and an unintended help from SkullKnight, his death destroyed the barrier between the real and the fantasy world bringing forth the Age of Fantasia.