I tried it, it's called a "mask"(It was called like that in Csro) when the monster dies a Rogue lvl 20 skills can use the skill and do a move and then there is an extra bag on the ground(do it when monster fall down on ground) and then you get a mask, use it by right mouse and voila into a monster, but when you make it in a SHARE PT you got a big chance that somebody else will get it(ask for it or rejoin pt)
I still wonder about the practical use of that skill. All I managed to do with it so far, is get a couple of laughs for changing into one of those green lvl 1 monsters while grinding in a party or have people run away in disgust, after changing into a wolf and begging them for food.