Story of marriage of Rishi Agastya (अगस्त्य) and Lopamudra (लोपामुद्रा)
This story is mentioned in Mahabharata Vana Parva – chapter 95-99, that section is called as Theerthayatra Parva (तीर्थयात्रापर्व)
Before the start of Agyatvasa, pandavas went for pilgrimage with rishi Lomash (लोमश ) & Dhaumya (धौम्य). When they reached near Agastya Ashram in Manimati city. Yudhishthira asked Lomash rishi – Why did Rishi Agastya (अगस्त्य) killed the demon Vatapi (वातापि) here on this place ?
Lomesh replied that this Manimati kingdom was ruled by Ilvala (इल्वल) demon and Vatapi (वातापि) was his younger brother. Ilvala has power that he can make anyone alive just by pronouncing his name. Ilvala misused this power to killed many brahmans, using his power he can convert his younger brother as goat and than cut & cook him, and feed it to brahmans using disguise. When Vatapi use to enter brahman’s body, Ilvala used to call his younger brother – “Please come Vatapti”, and his younger brother body parts used to come out of brahman’s body, by tearing it and thus killing them.
Once Agastya was going somewhere, when he saw his forefather hanging by their legs upside down in a pit. Rishi Agastya when asked them the reason, they told him that Agastya should get married and should have a child, than only they will be free from their current state.
Rishi Agastya decided to get married , therefore he created a girl taking the best parts of every species, and than gave the girl to King of Vidarbha. The girl was named as Lopamudra (लोपामुद्रा). After many years when Agastya came to know that Lopamudra has became a women of a marriageable age, so he went to King of Vidarbha and told his desire to marry Lopamudra (लोपामुद्रा).
After getting married both Rishi Agastya & Lopamudra went to Gangadwar ( Haridwar) and settled there. One day Rishi Agastya asked his wife to have sex with him, so that they can give birth to a child. Lopamudra denied this request and asked Rishi Agastya to provide beautiful ornaments, clothes and comfortable bed similar to queens, than only she will bear a child of rishi Agastya.
Rishi Agastya decide to get the desired wealth from a king, so he went to King Shrutarva (श्रुतर्वा), King Bradhnashva (ब्रध्नश्व )and King Trasadasyu (त्रसदस्यु) but none among them have surplus wealth to give it to Rishi Agastya. So they all decided and went to demon Ilvala to get the wealth.
But demon Ilvala (इल्वल) with his wicked intentions, he made his brother Vatapi as sheep and cooked his meat and fed it to Rishi Agastya. Even after knowing the intentions of Ilvala , Agastya ate the food. Ilvala (इल्वल) called his brother Vatapi – “Please come brother”, but he never came back, because with power of Rishi Agastya he cannot rejoin his body. Soon Rishi Agastya killed Ilvala (इल्वल) demon too and took his wealth.
When Rishi Agastya returned to his wife Lopamudra with this wealth , she was happy with rishi. Rishi Agastya and Lopamudra had a son named as Drdhasyu (दृढस्यु) or sometime called as Idhmavaha (इध्मवाह)