While casting this spell you perform an ancient ritual, calling upon a infernal to strengthen your pact. During the casting time of this spell, you and another willing creature within range must use the knife used in the casting if this spell to stab themselves, otherwise the spell fails. Both creatures take 1d4 necrotic damage, which cannot be reduced in any way. The maximum hit points of each creature is reduced by the same amount for the duration of the spell. 

Both creatures must remain within range for the duration of the casting time. During this ritual, the caster chooses a fiend of challenge rating 15 or lower. At the end of the casting time, both creatures must willing agree to the terms of an agreement, otherwise the spell fails. This agreement can be any set of rules or conditions to be followed by the creatures. For example, the pact could be an agreement not to attack eachother, or an agreement to attempt to steal a magical artifact if they ever see it.

Once both creatures agree to the pact, it is sealed by the infernal called upon in the casting of the spell. While the spell lasts, if either creature intentionally breaks any part of the pact, the infernal called upon is awakened. The creature is immediately teleported to the nearest unoccupied space of the creature who broke the past, the other creature involved in the pact is immediately alerted that the other creature broke the pact, and the spell ends. The infernal remains for an hour or until it kills the creature who broke the pact, after which it is teleported back to where it was before the spell was cast.

At Higher Levels. When you cast this spell using a spell slot of 7th level or higher, the maximum challenge rating is increased by 3 for each level above 6th.