The Ferula Gemini was a magical half staff about two feet in length that the demon Toth used in an attempt to destroy Buffy Summers. Giles referred to him as a "sophisticated Demon" meaning he was known for using tools.
He created the "tool" himself, the last ingredient to complete it being "pain", which he achieved by placing the device into a boiling cauldron with his bare hand.
The Ferula Gemini fired bolts of energy that physically split the personalities of any being that they hit into two separate entities, one with the strong qualities and one with the weak qualities. However, as the two beings were both "real" and technically one being, the death of one will kill the other.
The effects of the staff were both shown and explained:
- If Buffy had been hit by the bolt, she would have been split into two beings, one possessing all of the traits unique to Buffy Summers and the other possessing the traits unique to the Slayer alone; the Scoobies' assessment of this division was that "Slayer-Buffy" would have been virtually unkillable, but "Buffy-Buffy" would have been relatively easy to take out.
- When Xander was hit, his strong half demonstrated his more aggressive traits, such as his courage and confidence, effortlessly getting a promotion and a large apartment, in addition to his more impetuous qualities such as his impatience, while his weak half demonstrated his more passive qualities, such as his sense of humor and fun, in addition to being highly insecure and paranoid to the point where he believed that his other self was in fact a demon who was trying to steal his life and hypnotizing his friends with a shiny disc. Both Xanders, however, were highly protective of Anya.