Wikipedia
Ilvala and Vatapi were rakshasas, they ruled from Badami, formerly known as Vatapi, named after rakshasa king Vatapi. According to the Mahābhārata, Vatapi ruled the area along with his brother Ilvala. Legend has it that both rakshasa were vanquished by the sage Agasthya. Ilwala, the word means "absence of good qualities, of restless mind". Vaataapi means "one who drinks of all life” meaning "death". Once Ilvala invited a tapasvi Brahmin, played host to him with reverence and asked him to grant a boon. He desired to have a son equal to Indra. But the sage did not oblidge. Following this, his anger turned to hatred against all Brahmins and medicants. Ilvala and his brother Vatapi took a grudge against sages and hated them implacably.
Vatapi knew the art of transformation and had the power to change into any life form. Ilvala knew the ‘Mritasanjivani’ mantra to bring back the dead to life. They used these powers to loot and kill people. Ilvala would invite a brahman to a feast and Vatapi would turn himself into a goat. There he would serve the meat of the goat which Vatapi had turned into. Ilvala would offer the meat to the sages and later invoke his brother, Vatapi. After the Brahmin had partaken of the feast he would call his brother out by shouting "Vatape athragacha" who would rend his way back to life, tearing the belly of the guest.