Charybdis is one of several Greek monsters that appeared in multiple famous myths, such as "The Odyssey". She is often known as a swirling whirlpool of death that swallowed enormous amounts of water and anything that got caught in it.
She wasn't always this way though. Originally she was a naiad, a water nymph, and the goddess of the tide. She was the beautiful and an immortal daughter of the sea god, Poseidon, and the earth goddess, Gaia. In this form, she served her father as something of a conqueror in his quest for dominance of sea over land. Poseidon would create storms on the sea which Charybdis would ride onto the land, flooding and drowning towns, forests, and beaches.
She was so successful in her work that Zeus took notice of her on his land and grew furious. He transformed her into a sea monster, a giant beast with a monsterous mouth, and chained her to the bottom of the sea in the Strait of Messina. Three times a day her sea monster form would suck down water from the sea and spit it back out, forever continuing her duties of creating the tides, but swallowing anything that got caught in her whirlpool.
Charybdis famously sits directly across the Strait of Messina from another sea monster, Scylla. Together they make travel through the Strait a nightmare decision - to either be be eaten by Scylla or swallowed by the whirlpool of Charybdis.