Witches can (Though not all of them have to, and most don't) make alliances with extra-planar creatures.
The main forms are:
The Daemoniorum Pacem (Peace of Demons).
The Foedus Angelorum (Compact of Angels) .
And the Foris Vinculum (Bond of Outsiders) - a general term referring to alliances with other extra-planar creatures, such as the Kiin'Adu, Hunaal, or the Duun'Variin.
All of these have specific benefits, and all witches will be approached at the beginning of the game by a representative of each group. If they choose not to accept a deal with any of them, nothing further happens. If they do though, then they will have benefits and restrictions as negotiated with the DM.
Let's take a look at some examples of each of the three:
Daemoniorum Pacem: Marianne of Lamir:
She was approached by a demon named Nantuana, who offered her visions of the future and increased magical ability, in return for the sacrifice of a dog each week. The dog must be burned alive in Sessir Oil, and then eaten by her companions.
All spells are cast as though she is one level higher than she is.
She can prepare an addition spell each day.
She can use Precognition (as the spell) once per day at no cost.
Foedus Angelorum: Rothil Oorlog:
He was spoken to in a dream by an angel named Tospirxatael, who gave him special spells and powers in return for converting 'heretics'. (One follower of another religion per month must be converted, and brought back to a church of Xomut and Rezaz for purification)
Angel's Favor: Reroll any failed roll once per day.
Rewards of Faith: Once per day, he can heal 2d20 HP. Additional HP over any damage taken will be used as temporary hitpoints - they will protect the character from harm, but cannot be restored. This rule is subject to the Big D30 Rule, with the following modification: The player will roll 2D30 instead of replacing both.
Foris Vinculum: Too numerous to list here, as it includes all other extra-planar creatures.