Martanda exists because of a few lines in a single hymn of the tenth book of the Rigveda, the same hymn that says Daksha is born from Aditi and Aditi from Daksha. Aditi bears eight sons. She brings seven of them forward to the gods and casts the eighth away. Then, in the next breath, she brings that eighth back, for birth and for death. The passage does not explain itself and it does not apologise for the reversal.
The name is the part everyone starts from. It is read as a compound of marta, dead or mortal, and anda, an egg, so it means the dead egg or, in some renderings, the dead bird. That reading is old and it fits the story, because what Aditi throws away and then recovers is something described as lifeless which nevertheless becomes a beginning. It has not gone unchallenged: the compound is transparent and the sense of it is not, and every translation of the hymn has to decide how much of the etymology to let into the line.
What the passage is doing is easier to see than what it means. The Adityas of the classical lists are twelve. This hymn has eight, and elsewhere the same collection has six and seven. So the casting away of the eighth sits inside a body of poetry that had not settled how many sons Aditi had, and it may be a way of accounting for the difference between two counts rather than a story about a person at all. That would explain why Martanda has no career. He is not a god who lost his cult. He is a name that a single hymn needed and no later text found a use for, except as a problem to be interpreted.