When Shapechange is cast, the wizard is able to assume the form of a powerful creature.
With this spell, a wizard is able to assume the form of a powerful creature (mind flayer, iron golem, greater wolfwere, earth elemental, fire elemental, or giant troll). The spellcaster becomes the creature he wishes and has almost all of its abilities. Thus, he can change into a mind flayer to stun his opponents and then become an earth elemental to crush his foes. These creatures have whatever Hit Points the wizard had at the time of the shapechange. Each alteration in form requires only a second, and no system shock is incurred.