Samuel Agreste-Toulon has gone mad. VERY insane because of the grief of losing Rosie and Amelia. Rosie when she died in the universe they fled to, and Amelia in the collapse of their old one.

So...he unifies the Goat and Snake Miraculous, and unleashes their full power, knowing full well he's made of Alcatraz Cells and this will corrupt him.

And he corrupts into the form of a giant snake, with giant Goat horns.

THE HORNED SERPENT STILL LIVES.