I have very little background in Buddhism; five minutes of internet research indicated to me that Mara (http://en.wikipedia.org/wiki/Mara_%28demon%29) seems to be identified with Dairokutenma-ô, largely because they're both names for the demon-king who got on the historical Buddha's case back in the day. More to the point, Oda Nobunaga at one point apparently signed his letters (!) as Dairokutenma-ô (http://en.wikipedia.org/wiki/Mazoku).

I may revise it later, if I can get to a library and do some research on it, but for now 'Mara' is easier to type and hopefully gets the point across to that small minority who read the manga and know something about Buddhisms. Whereas 'Dairokutenma-ô' is highly culturally specific, and people who know Japanese presumably aren't reading my translations. =)