Mahirs... hmmm..
Well, they could be a race from a unmentioned part of the Myth World, like the untamed lands or something...
Still, these beings are fair to say magical. holding a living being and taking its live energy suggests a undead being of some sort.
They could indeed be shadows, driven to wander the known world when their living bodies where killed.. If so, i'd say they are shadows of shades or fetches.
Since Shades are undead beings of great sorcery and fetches are summoned from another world, it is possible that their shadows could disconnect from them and go kill as they please... Afterall, Fetches are not from this world and who knows with Shades?.....