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Monster classes do not follow the same guidelines that Pathfinder normally uses for playing monsters as PCs. Instead, the monster classes introduced in this book work similarly to any other class, with a few key exceptions:
To use a monster class, a character must take it as their initial character level. Each monster class has an associated race; instead of selecting a race, the character gains the benefits and drawbacks of the race associated with their chosen monster class. A character normally cannot take levels in more than one monster class. A character may multiclass freely between their monster class and other classes, and is not required to finish their monster class. A character with relatively few monster levels might be a runt compensating for their relative weakness with training, a freak of nature, or maybe even a magical experiment gone wrong. As usual, a character cannot normally have levels in more than one monster class. Some monster classes have abilities that have been split across multiple levels or changed to a similar ability that increases in power as the character gains levels. When a character gains one of these abilities, they are not gaining the same ability as the monster itself. While monster classes are based on monsters from the Bestiary, they are not exact replicas. When a character completes the monster class, most of these abilities should function as the base monster’s ability, but this will not always be the case.
Some monster classes have abilities that reference a character’s Hit Dice. For the purposes of abilities granted by monster classes or feats in this book, a character’s Hit Dice is the sum of their racial hit dice and all class levels. Some monster classes, such as the astral deva, allow the character to trade their native spell-like abilities for a spellcasting progression. If such a character later gains levels in a spellcasting class of the same type (arcane, divine, or psychic), the character can opt to continue the progression of their monster class, rather than gaining spells from the new class. The character gains new spells per day and an increase in caster level and spells known as if he had also gained a level in their monster class (even if the monster class does not have that many levels). This replaces the new class’ spellcasting progression in its entirety, including the ability to use spell completion and spell-trigger items as a member of that class. A class that only casts spells at a later level than 1st (such as the ranger) only stacks the levels where it gains spellcasting.