When found, this weapon appears to be nothing more than a very elaborate hilt. It has a silver cross guard in the shape of a lightning bolt, a grip wrapped in blue-dyed leather, and a pommel circled with brilliant blue sapphires. One might at first suspect that the blade has simply broken off, but any cursory examination of where the blade ought to be will reveal no trace that a blade was ever there.
If the sword is held aloft, a bolt of lightning will arc from the sky towards the hilt. If the bolt reaches the hilt, a 3ft long blade of jagged ice will slowly begin to form over the course of 1 minute. However, if there is anything in between the hilt and the sky (such as a roof, or a monster) then the lightning will strike that instead, dealing damage as normal. (6d6 damage, save v. breath for half). Lightning cannot be called for while the blade exists, nor can it be called for more than once per minute.
Note that while underground, holding the hilt aloft will cause a lightning bolt to strike the surface directly above you, potentially causing harm the player will not be aware of immediately.
Once the ice blade has formed completely, it functions as a longsword. Upon a successful hit, however, the ice blade shatters into a cloud of snow. Creatures with 5HD or or lower are frozen completely solid, and could easily be kicked into tiny pieces. Creatures of 6HD or greater take 6d6 frost damage, and are frozen to the ground until they can make a successful strength check to dislodge themselves. In particularly hot environments, a cumulative +2 each round would be appropriate.