Spirit Pact is an Extra Skill chosen at character creation or class changing, and it is exclusive to the Summoner Class. It allows summoners to transform summoned monsters as adventurers, letting you use it in place of a normal adventurer created at the Training School. All monsters reborn as adventurers through Spirit Pact will carry over the following various attributes from 'monsterhood':
- Status & Turn Recovery
- Attack & Defence Attributes
- Weapon Breaks
- Spells, and they can actually learn additional spells from the same tier & class
The following are not carried over:
- Level; reborn monsters restart at Level 1
- HP; goes to the default 8 HP
- Race (Ghost, Giant, Mythical, etc.); will typically convert to a Devilish, but also occasionally Human and Elf
- Equipments nor any items in bag or servant 'pouch'
- Most skills will be lost, some may be retained and a new one may pop up
- Number of actions
- Enemy-exclusive spells (e.g. Draw)
The class of the newly-reborn monster is largely determined by the skills of the monster before Contract. For example, monsters that have Improved Beheading Rate will likely be of class Ninja when reborn. It is helpful to consult the Monster Encyclopedia for more info. A lot of monsters will be reborn as Fighters.
Bear in mind to differentiate between a monster's Race and its Class. The former is important for determining Double-Strike, and the latter is relevant for a character's general stat growth and spells/skills learned. For example, a Phantom is of the Thief race (not to be confused with the Thief class) but when reborn as an adventurer it is of the Ninja class.
Spirit Pacts have a cost of 1 SP, which can only be replenished with the more expensive rest options at the Hosbarn Inn. Note that a summoner may use Spirit Pact on any summoned monster, not just the ones they themselves summoned. This allows you to use it on the monsters from higher level summoners to whom you've not assigned the Spirit Pact Extra Skill.
An important (and oddly unexplained) point to remember is the fact that most of a monster's resistances and attributes are lost should it Class Change at the Training School. It is because of this that it's often wise to keep its given Class.
The general rule of finding which monster to use Spirit Pact on is to find those with extraordinary resistances and attributes, like the spell-immune and ailment-resistant Sign and Mirror Ghost, both found in Afum Zar Light Cave.