- Georgius of the Rainbow.(src)
Contract of the Rainbow's formation
Arcus Pactum (アルカス・パクトウム Arukasu Pakutoumu, lit. Arc of Covenant in Latin), or more commonly known as Contract of the Rainbow (契約の虹 Keiyaku no Niji) is the forbidden trademark spell of Georgius of the Rainbow. It is the ultimate spell that enabled him to imprison the immortal Witch of Babylon, Lieselotte Werckmeister.
From how it was used, Contract of the Rainbow is most likely a dimensional-affecting-spell. It has the ability to shatter reality of its target, greatly weakening or even killing the user's enemy off from reality. The use of Contract of the Rainbow, as stated by Shiori Momono, can cause the root of reality to be shattered beyond repair, which is perhaps the reason why it was forbidden in the first place.
Since it is a powerful spell, the user is almost guaranteed to face death right after execution. This drawback can be overcome by providing the user with an abundant amount of magical energy, however.
In the anime, the spell was used by all six members of the Black Knights to seal off Lieselotte; the price was that they had to take on their current grotesque forms.