The Netbook of Witches and Warlocks
By Timothy S. Brannan and The Netbook of Witches and Warlocks Team
Full netbook can be found on the followng website
During the daytime, this longsword functions as a +2 flaming burst weapon. During the night, it functions as an icy weapon, with the additional ability that whenever it strikes a critical hit, the victim must make a will save DC 15 or come under the effects of a Sleep spell.