mara is a Buddhist demon personifying temptation, from the Sanskrit मार māra "killing, destroying, death, pestilence". This seems to be derived from the verb mr̥ "to die". (This is the source of the Doctor Who mara.)
The Proto-Indo-European root is *mer- "to rub away, to harm". (Pokorny has two roots: "to die" and "to rub", the AHD has one). The prefixed and suffixed form *n̥-mr̥-to- (with the negative prefix *n̥-) became Greek ἄμβροτος ambrotos, as in ambrosia. This PIE form is also the source of Sanskrit अमृत amr̥ta "not dead, immortal" (perhaps the source of the name of the city of Amritsar), and Latin immortālis.
mara is also a demon in Scandinavian mythology. The Old Norse word mara is from Proto-Germanic *marōn "goblin", from PIE *mer-. Old English mare "goblin", from the same root, combined with night to form nightmare, originally "a demon that affects sleeping people".