As far as I know it comes in two forms : either as one of the golden horns found in some random chests or as a god given item for those lucky of us that worship Ixalovh

From the archetypes it looks that both horns cast Siren Call, which works like charm monster.
At the moment the god given artifact horn casts magic bullet, however I had limited success charming mice with a golden horn of Sirens, and I say limited because...well, mice is the only thing I could charm

My question is: does anyone have any idea what the roll is against when trying to determine if the monster will be charmed ? A part of the roll is overall monster level, but which skil is used(if any), magic item ?