Harry Potter: 10 Hardest Spells To Master
4. The Animagus Spell
Though three of the four Marauders, James, Sirius, and Peter, were all Animagi, as well as Minerva McGonagall and Rita Skeeter, the ability to change into a particular animal at will does not come easy. This is something that must be learned, and the entire spell requires a combination of actions, a potion, and an incantation.
For an entire month, a witch or wizard must keep a single Mandrake leaf in their mouth, having to start from scratch if it chewed, swallowed, or spit out. After a month, the leaf goes into a potion. Every single night between making the potion and the next electrical storm, which could potentially be years away, the witch or wizard must chant the incantation Amato Animo Animato Animagus without fail.
When the storm finally comes, the witch or wizard must drink the potion and site the incantation once more. This is an enormous amount of effort, concentration, dedication, and skill for a transformation that, if done incorrectly, could have horrendous consequences.
The subject doesn't even get to choose what animal form they take on. Instead, this is based on personality traits much like the Patronus. This means that Peter Pettigrew went through all this, just to find himself with the ability to turn himself into a rat!