It's a pity that the Atlantis team didn't know their mythology better or they would have been better prepaired. Asuras are divine beings, not quite the same thing as demons in Hindu mythology, but also not exactly friendly cuddly folks either. Here's desciption I found.
Asuras
by Stephen T. Naylor
The asuras are usually called demons, but this is not a terribly good translation, for it denotes a sinister quality which is not always in the character of these beings. Strictly speaking, the asuras are the powerful beings who are opposed to the devas. In early Vedic times, they were originally another class of gods, but who were incorporated into the new pantheon. Such important Vedic gods as Varuna and Mitra were classified as asuras. By the end of the Vedic period, however, the asuras had attained their more demonic role. Certainly some of the asuras were quite evil, such as Vritra, but some of them were also as pious if not more so than some of the gods. They would, at times, even be more powerful than the gods, forcing them to flee in the face of their power until some way of dealing with them could be found. On the other hand, in certain instances they were known to work alongside the gods for a common goal.