Basil and the human army he led were in a situation where they were bound to be devoured by the sea. The unstoppable, massive whirlpool that Charybdis made was gradually pulling them to its eye.
As they were in Scylla's domain, luckily, they only needed to defeat her to get out of the situation; by then, Charybdis' whirlpool wouldn't be able to do anything to them as it had lost its effect. Of course, this wasn't an easy thing to do.
Scylla was the embodiment of the consciousness of a killing machine that was made by Poseidon. She was a creature that deserved to be called Monster: she was an entirely different creature from Demons and Magic Beasts.
She used Demonic Mana, but she could utilise it as good as any Demon Prince, which added to her level of danger. Basil needed to face this kind of abomination while racing against time; he had to kill her before the whirlpool devoured them.