Madhu and Kaitabh were born from the ear dirt of Lord Vishnu. These demons were very powerful and had a boon to choose the time and the way of their death. At that time, there was no land and no humans were born. There was only water all around.
Once these demons saw Lord Brahma sitting on Lotus flower. They planned mischief and challenged Lord Brahma to either fight or leave Brahmlok.
Lord Brahma, the creator, was reluctant to fight as it was against his deed. He then went to Lord Vishnu for help. Lord Vishnu was asleep. Lord Brahma tried to wake him up but was unsuccessful.
So, he went to Goddess Bhagwati and explained the whole situation. She understood and agreed to help Lord Brahma. She woke Lord Vishnu using her spiritual powers.
Upon learning about the whole situation, Lord Vishnu became angry. He then fought with Madhu and Kaitabh. The fight continued till 50,000 years without any conclusion due to the boon given to Madhu and Kaitabh.
Then he thought of a plan and asked both the demons to wish anything they want from him. The overconfident demons then told Vishnu to wish for anything. They got blinded by their power.
Lord Vishnu then asked for the boon :
O demons, grant me the boon that you will die from my hands
The over-smart demons granted him the wish which marked the beginning of their end. They soon realised their mistake.
To revert their destiny, they then asked Vishnu that they can only be killed at a place where there is no water.
They did this intentionally as at that time, there was no land, only water could be seen.
Hearing this, Lord Vishnu grew bigger in size and pulled the demons on his laps which served as a land. He then killed the demons.
This is the complete story of Madhu and Kaitabh according to Durga Saptsati. This story teaches a lesson that the devil may be very powerful and may last for long but will definitely meet the destiny of death one day.
Truth is eternal !!