IMO, the only way most Darkmantles would "use" a magic item or other treasure ... is by collecting a pile of glittery treasure (and glittery junk) into the BEST place for it to make an ambush attack. ^_^
you could go with the whole dark master route. The darkmantle is a pet of someone who equpped it with a few things to pump it up.
Then when your players say "that thing is too dumb to put on magic items." you can give the characteristic dm smile and say "you know your exactly right"
