This particular type of athame, referred to as a Power-Sucking Athame, has the ability to absorb the powers of whomever is stabbed with it. The powers are stored within the blade until they are extracted. This type of athame was seen only once during the series, in episode "Witch Wars", where the Charmed Ones needed the books from Magic School to discover the origin and purpose of this magical weapon.
A demon couple hosting a demonic reality TV-show called "Witch Wars" used many of these athames for the purposes of their game show. Phoebe Halliwell and Leo used it to go after Clea and Leo vanquished her with the athame.
Phoebe then took Clea's powers from the athame and went after Corr, vanquishing him with an energy ball and then using the powers to vanquish a bunch of demon fans before Paige stabbed her to remove the powers.
Appendices
Upon finding this athame, the Charmed Ones tried to identify it with their Book of Shadows. Although they did not find anything in their book, they found an entry on power-sucking athames in a Magic School book. The entry reads as thus:
Power-Sucking Athame entry in a Magic School book.
The unique design of the
Athame pictured above is used
as a power absorbing channel
from its victim. Any magical
individual killed by this weapon
has their powers transferred and
stored within the blade.
Notes and Trivia
Although the entry states that a being has to be killed for his/her powers to be stored within the blade, being stabbed only seems to suffice.
It is unknown if this athame would work on non-corporeal demons (such as the Water Demon and the Shocker Demon) or demons who feed on magic.
