An interesting story from Srimad Bhaagvatam
Narakasur, the demon king of Pragjyotishpur, now a days known as Kamrupa in the western state of Assam, attacked on the surrounding mountain kingdoms and conquered the entire Brahmaputra valley; consisting of current west Bengal, Bhutan and parts of Bangladesh. The defeated kings were killed, their armies destroyed - queens were captured, women and children were enslaved.
He was evil, who had become extremely powerful with the association of another asura named Baanasur. Now, drunk with power, he wanted to conquer Indra, the king of Devas.
Indra knew he could not defeat Narakasur so he approached Lord Krishna for help.
Lord Krishna, along with his wife Satyabhama and elder brother Balram went to defeat Narakasur. Krishna attacked from above on ‘Garuda’ (Krishna’s vehicle) while Satyabhama and Balram, along with their army, fought on the ground. Narakasur’s army was destroyed in no time, and its chief commander Mura was killed by Krishna*. Before he could reach to Narakasur, Krishna received a news that Dwarka (Krishna’s own kingdom) was in flames.
He said to Satyabhama “Dwarka is burning. I and Balram must go back”.
Satyabhama was shocked. “Do they want you to come back to save them? They don’t listen to you anymore. They are always fighting with each other. They don’t even want you”.
“It does not matter whether they want me or not, I must try to save them. It is my Dharma. But you must stay here to finish what we started”
Satyabhama bowed her head to her husband “I understand and I accept. What must be, must be”.
Krishna turned towards Balram and said “Dwarka will be burnt before the dawn. Enemy is at the gate. Our family, our people are fighting with each other. They will soon destroy each other. We must leave now.”
Balram Could barely believe what Krishna had said.
So it was all coming to an end then - this yuga, this Avtaar?
The wheel of time had turned and another era was coming to a close.
“You knew my work here will be done soon”. Krishna said.
"My time here is approaching an end. You know that this form I took in this yuga, cannot last forever. It serves a purpose and that purpose is reaching the end of its span".
“Is Dwarka really going to be burnt? Is everything going to be destroyed…our families, our dynasty, our people?”
“Yes, this is their fate”
Balram sighed. “Of course you know. You always know.
But what about here? There is so much yet to be done here.
The ‘Evil’ Narakasur is still alive.
“Satyabhama will deal with Narakasur and handle everything here” Krishna said. “It’s time for us to go”
Balram sighed again.
“But I still think there is so much more to do here, with this world, with these people, with this time.”
“There will be other worlds, other people, other times”
“Yes. There will be.”
Balram thought there was no point in arguing further.
Even Krishna had his limitations.
Even Bhagvaan; incarnation of god could only do so much.
He summoned Pushpak (his vehicle) and followed Krishna.
(Source: Srimad Bhaagavatam, Lord of Vaikunta by Ashok Banker, Google and few other books)
*Krishna is also known as Muraari for killing Mura