Venta had been exploring this new territory, though guilt filled her half-heart. Stupid. You should've know Vanitas would've done something, puttanna! Her ears were visibly down and she was distracted. That's when the Cartellian attacked.
Before she knew it, she was pinned on the ground by the melanistic lion, before being stabbed, and having removed what darkness she had gotten from her mental illness.
After it joined with Vanitas, he felt slightly bad, wrapped his other half in a robe, and placed her near the Daremo homeland to live out her last days in peace before she'd eventually slip in a coma. With that, the 'demonic' lion left.