He came to earth from an undetermined plane of existence when Abraham, father of Ezekiel, summoned M'Nagalah to earth.  M'Nagalah attached himself to Abraham in the form of a tumor.  He continued to grow until he consumed Abraham, taking his physical mass into his own.  M'Nagalah continued to consume whatever living creature would pass by it, hoping to attain his full mass before the cosmos re-alligns with itself in order to take over the universe.  M'Nagalah met up with the Swamp Thing and decided that it's mass would be enough to reach his full size.  It tried to consume the Swamp Thing, but the Swamp Thing escaped, leaving M'Nagalah in a collapsing mine.  M'Nagalah is assumed dead, despite being last seen attached to the arm of a townsman who lived by the Mine. 

M'Nagalah has claimed to be the one to decree the birth of life on planet Eartth, as well as give human beings the desires and abilities to give into "mindless violence".  He has also claimed to have inspired some of histories greatest writers and poets.