This scarab is absolutely indistinguishable from any other amulet, brooch, or scarab. 

When displayed and a command word is spoken, all other creatures within a 2" radius must save versus magic at -2 (and -10% from any magic resistance as well). 

Those failing the save are completely insane for 9-12 rounds, unable to cast spells or use reasoning of any sort (treat as a confusion spell (q.v.) with no chance for acting in a non-confused manner). The scarab has 9-16 charges.