so here is what i can tell from literally 30 different playthroughs of the game. this thing is 100% guaranteed. if you have not gotten it, you always get it from at least one mimic, the catch is, sometimes it wont drop from the last one you open. from what i can tell, this is a glitch where the game actually tried to drop it from an earlier mimic, but either didnt, or it did, but you just... missed it. that is why people sometimes find it where there last dead mimic was, or sometimes halfway through the game at a random mimic's location. i have never failed to get one in a playthrough, and i have never grinded for it. If one does not drop from the last you kill, check it again, and if its not there, check the other locations, most common ones appear to be the one near Pontiff Sulivan, and somewhere in the Profand Capital.