What happened when Mrs. Dobbs took Percy into the museum alone?

she turned into a monster and attacked him, but the pen that his teacher had given percy turned into a sword in his hand and he was able to defeat her

While at the museum, Percy's math teacher, Mrs. Dodds, takes him inside and turns into a monster, a Fury, one of the minions of Hades. after this scene, Grover is forced to reveal the secrets behind Percy's origin. He fears that Percy has seen the true form of Mrs. Dobbs.