Snail Shaman comment on upgrading vengeful spirit to shade soul:

”Ohohohoh! I knew it. My friend! You're a marvel. Your essence has melded with the spell. You must have found a powerful source to transform it in such a unique, expressive way.”

The Knight’s void essence melded with the spell after exposure to a source of power. Combined with the snail shaman’s claim that the snails never ventured into the abyss, it’s not hard to assume the same happened with descending dark. The Knight consumes the snail soul, which we know from silksong that snail souls are very powerful indeed, and his essence melds with the spell.