It's an item in Pokemon Gold/Silver/Crystal that is a hold item for it. It doubles your Base Attack but it makes you confused. It dissapears after the use.
To obtain it, go North of ceurelean city on route 25 there is a long house and a Field behind it in which a guy is stood go talk to him, he will then mention a cave from previous games of Pokemon then pause and tell you his item finder is responding. at this point exit of it turn left and you will see some water. surf down till you see the man in the field and use your item finder and press the A button anywhere horizontally in line with this guy you will soon fine the item berserk gene.