A Dispel Scroll is inscribed with a powerful anti-magical invocation. When it is read out by a Wizard, the effect is to drain away magical power and weaken a spell that has been cast. As soon as a spell has been cast, any Wizard who has a Dispel Scroll can read it. This automatically dispels the cast spell, no dice roll is required.
Reading a Dispel Scroll will bring any spell cast by the Wizard reading it to an end. To put it another way, a Wizard who has a spell in play will automatically cancel it by reading a Dispel Scroll.
As with all scrolls, Dispel Scrolls are not unique items - they are prepared by a Wizard prior to battle and it is quite possible for several Wizards to carry Dispel Scrolls, and for a Wizard to carry more than one. However, only one can be used at a time.
Note that a Dispel Scroll will not help if the spell has been cast with Irresistible Force. Any spell that is successfully cast with two or more 6s is Irresistible and no Dispel roll is permitted.