The bats of destruction were a magical weapon created by Lena LaMarelle. They were initially metal baseball bats enchanted to attack a named object on voice command, but then Lena branched out into swords and axes of destruction, too. In Of Sorcery and Snow, the bats are used against Lena by Searcaster. The attempt backfired, and let's just say that there were a lot of villains with metal shrapnel in their skin the next day.
The bats and other weapons of destruction have been used against Lena and her friends more than once. One attempt was mentioned here, but the other,in Of Enemies and Endings, was more successful, cutting off Lena's hands.
Lena also has swords and axes of destruction, both introduced in the fourth book.
the weapon that cut off Lena's hands was actually an ax, not a bat.
In case of attempts to turn the weapons against her, Lena added voice recognition software. Unfortunately, a recording worked to turn them on, so to speak, too.
Lena basically took out a whole pack of wolves during Miriam's Tale with these.