The Berserk Staff (バサークの杖 Basāku no Tsue) is a long-range Staff that is used to make hostile units go berserk, making them uncontrollable and attack any unit within range, be it friend or foe, at the end of its faction's turn. Thankfully, they are inaccurate, but are usually enemy-only staves.
This particular staff is arguably one of the most annoying staves in the series, due to the fact that it will cause your own units to attack you. Because the affected unit cannot trade or be rescued, getting hit by this staff nearly guarantees that the unit will either kill multiple allies or rush into a crowd of enemies.
However, it can also be one of the best staffs, as an enemy unit powerful enough but with low resistance can attack enemy units, softening them up for your units before going in. For example, if you have a Berserk Staff, and use it on a Draco Zombie, it can be very devastating for enemies (Wretched Air, of course, dishes out moderately heavy damage on units). It can also be used on bosses or core enemy units to destroy other units nearby.
The Restore Staff can be used to counter its effect.