Spelicastiolarius is a spell that is probably one of the most complicated in the entire annals of magical history.
Back in ancient times, magic was shapeless. Finally, a wizard named Oglamius McAster made the first wand. McAster only found one dillema: You couldn't use a wand without any incantations. Finally, after many years of research, McAster discovered the charm Spelicastiolarius.
Spelicastiolarius is a spell that creates a spell. The wizard or witch just has to think what the spell will do while they say: "Spelicastiolarius <Name of the new spell>". This requires a great amount of concentration and skill. After someone died of the effort of creating the spell Avada Kedavra, the spell was banned for common use. But, if a wizard does a lot of research, then he/she can create his/her own spell.