Long long ago, there lived an king called Narakasura. He wasn't always evil, the son of Mother Earth and Lord Vishnu's incarnation Varaha the Boar, he was initially pious. He overthrew the last danava of earth to establish his rule over all of Kamrup (modern day Assam). He also built a new capital, Pragjyotishpur (said to be today's Guwahati). His initial benevelonce saw him getting a boon of a extraordinarily long life. He used this boon to plunder Swarga time and again, making Indra penniless for a zillionth time. While Indra was used to this 'regime change' the present situation was worse than before though. Earlier Indra would run to Lord Vishnu and the Lord would take an avataar and destroy the demon. But the that time avataar of the Lord, Krishna was as anti-Indra as Narakasura. Krishna had stopped the worship of Indra in households (remember the mountain lifting act? that was to protect Gokulwasis from Indra's reaction post this decision). Also Krishna had recently claimed that everything the Samudra Manthana produced (remember the cows, jewels, godesses, etc i mentioned yesterday) belonged to him. Indra was hardly in a position approach Lord Krishna.
But then Narakasura dug his own grave. He started plundering neighbouring kingdoms, not just for their wealth but also for their women. These kings, who had an ex-boy scout like bonhomie, approached the western-most (and thus one of the few as yet unaffected by the onslaughts of the first Assamese terrorist ever) of their club, Vasudeva Krishna. Always ready to uphold righteousness, he agreed. Thus the battle began.
Krishna being God-and-all, the battle was over soon. Krishna was at his impressive best, fully armed what with his weapon Sudarshana and mount Garuda. The Lord's 108 wives too turned up for moral support. After defeating Tamra, Antariksha, Sravana, Vibhavasu, Vasu, Nabhasvan, Aruna, Mura and finally Narakasura, Lord Vishnu proceeded to install Naraka's son as the next king (colonies separated by 100s of km can be very difficult to manage, something our not-so-friendly neighbour realised only in 1971). This event is celebrated every year as Naraka Chaturdashi. Yes thats the festival we have today!
He decided to return all the spoils of war to their rightful owners. These included strange trinkets like Aditi's ear-rings, a garland of vaijayanti, Varuna's umbrella and the rock that was the peak of Mount Mandara (now wasn't Narakasura weird to have collected all these?). But there was one problem when 16000 of the 'returns' were disowned. These were the women Narakasura had abducted; no one save the the 16000 and the Lord himself believed in their chastity. The Lord did the only thing an honourable man would - he married them all.
The Lord, his army and 16108 wives proceeded to hand deliver the stuff that belonged to Indra. Indra and his charming wife Sachi played the perfect hosts. They even showed the guests the grand courthouse Sudharma and the greatest of the Kalpavrikshas (wish-fulfilling trees) - Parijata. The Lord liked the former and his favourite wife (I know that sounds strange in today's monogamous world!) Satyabhama liked the later. They uprooted both and headed home. Indra, the fool that he is, decided to battle the Lord. He was, of course, defeated. He then famously declared that 'He always wanted to lose to Lord Krishna and it was an honour' - the most pathetic oneliner a loser ever gave. The courthouse and tree were then installed in Dwarka, where they remained till Krishna's death, when Dwarka itself got washed away.