Very confused about this skin

i recently downloaded a skin for arthas, a fellow user of this site offered to alpha
out the sword per my request. when i mailed him the skin that needed alphaing he sent me this message with an attached BLP titled Froustmourene. "ok if you want to get rid of Frostmourn then use your skin and put in
my
invisible frostmourn skin." WTF does that mean?! very confused. please help anyone.