There are prestige classes that apply or effectively apply a template to the character that attains a certain level.
To clarify, I am not asking about other "Apotheosis" prestige classes, those being prestige classes that change your creature type without applying a pre existing template, such as the Rimefire Witch.
Prestige Classes I have found that apply templates:
The Dragon Disciple from the Dungeon Master's Guide:
At 10th level, a dragon disciple takes on the half-dragon template.
This class does not apply the template all at once, merely gives you stats, natural armor, breath weapon, natural weapons, and wings building up to those equivalent to the template, giving you the remainder of your ability boosts at 10th level, then telling you that your character now has the Half-Dragon template.
Walker in the Waste from Sandstorm:
On reaching 10th level, you learn to apply the secrets of waste
preservation to your own body, becoming a dry lich. You must undergo
the Sere Rite, overseen by another dry lich, which includes preserving
your flesh, removing your organs and storing them in special canopic
jars, and imbuing your body with foul magic to make it undying. See
the dry lich template, page 155
This class simply gains the template upon reaching the appropriate level.
The Dragon Devotee from Races of the Dragon is much more specific about the template acquisition process:
At 5th level, you gain all the abilities of the draconic creature
template (see page 74) that you have not already gained as dragon
devotee class features—except that you do not apply the template's
level adjustment. Specifically, you gain the dragonblood subtype in
addition to any subtype you might already have. You also gain
darkvision out to 60 feet, low-light vision, and a +2 bonus on
Intimidate checks and Spot checks.
In the case of the Dragon Devotee and the Dragon Disciple, the prerequisites of these Template prestige classes set the precedent of not already having the template that will be granted by the class. (In the case of the Dry Lich, Walker in the Waste is required to obtain the template at all.)
If there are any prestige classes that "Basically" apply a template (In short, a prestige class that grants identical bonuses to a pre existing template, but does not explicitly state that it is doing so.) They are also in line with what I am looking for.
Best Answer
These are the classes that I can find that reference a template directly. Classes that “effectively” offer a template are trickier: it’s not always clear what counts, and in any event, there’s nothing to easily search by. I don’t have a plan for tracking those down at the moment.
As predicted, almost all of them gradually build up the benefits of the template, which is granted “properly” by the capstone. The only exceptions are Vadalis beastkeeper—which is a weird case—and the wild-shape-only templates from blighter and planar shepherd. Well, and the Dragon Compendium mountebank, but that’s a base class and the feature in question is quite possibly the worst in the entire edition, and in any event Dragon Compendium is by Paizo and not Wizards of the Coast.
Prestige Classes
Base Classes
Not requested, but there are a few