Mahatmas are beings who were once human and rose to immortality by tapas and help and guide the humanity. Here the Mahatma narrates his birth and growth towards immortality. When he was a human even then he cherished high thoughts, had a thirst for knowledge, a wide compassion, a sense of power for so many lives till he reaches the highest of human knowledge. Then the great Vyasa guides him to accumulate all the necessary knowledge Kuthumi has gathered so far in all the lives. Then Kuthumi learnt Hathayoga and Rajayoga and other required endeavors and experienced enormous power. At last he was commanded to find out Krishna and offer all that he gained. As Kuthumi was entrusted to save the knowledge for mankind, and when krishna would not be there physically in this world, there would be dense darkness till the next avatar comes. At that time Mahatma Kuthumi can save the mankind by this great knowledge. This was the task of Mahatma and it is narrated beautifully in the poem. The last of the Avatar, Sri Aurobindo came after so many long and dark phases of mankind after Krishna.
Night is a shadow and not some original Truth. That is why it has not the ultimate absolute Power.
About Savitri | B1C2-10 Condition of the World (p.18)