Charybdis

Charybdis (暴風大妖渦) is a gigantic one-eyed mindless blood-thirsty calamity-class Monster, heralded as the Ruler of the Skies. Born from a cloud of Veldora's Magicules, it can be said to be part of Veldora's "family," similar to Rimuru as it is also called to be Veldora's child. As it has no solid sense of self, it operates solely to satisfy its lust for destruction.

Since it is a spiritual life-form it lacks a physical body and thus requires a host for the sake of survival. It is also strongly resistant to magic attacks due to its extra skill "Magic Jamming." As it is originally from the spirit world, it is also accompanied by a couple of Megalodons who are strongly resistant to magic attacks as well. Though Charybdis is said to be a Calamity-class monster, in reality, it is no weaker than a Disaster-class monster (i.e., at the level of a Demon Lord) since it wields a fraction of Veldora's power. The only reason it hasn't been considered as one is due to its inability of rational thought.

Background
Born from the malice and violence of Humans, Charybdis entered a cycle of spreading widespread destruction and death to whatever caught its interest, before being subdued and killed, and eventually resurrecting a few centuries later. Seeking to end the cycle, the Hero sealed Charybdis away inside a dimensional barrier hidden in a cave somewhere in the forest of Jura, with its location being only known to the Hero and the Dryads.

Trivia

 * The name Charybdis comes from a sea monster in Greek and Italian myth.
 * Charybdis in all technicalities can be considered as something akin to that of a sibling to Rimuru Tempest due to her having also been born from the Magicules excreted by Storm Dragon Veldora.