The act of finding an recruiting a familiar is a time-honored tradition of mostly the arcane casters. It is part of their schooling to venture out and find and recruit a creature that will continue to serve them for a wide variety of magical rituals, endeavors, and even adventuring. For such Wizards the options usually entail either going out into the world and finding a denoted sub-creature to contract, or summoning a lower-ranking creature from a separate plane. The very process depending on what creature is hired will differ. Most creatures will accept some conditional work either for a short period of time or a longer lasting term. The very act of creating a familiar-pact requires usually a magic circle alongside various offerings and, of course, interacting with said creature. If the creature is too powerful for you (mechanically a CR higher than your character level), becomes nigh-impossible to contract the familiar. The stipulation can be overturned if a familiar is granted unto you by a higher power, such as a patron. In this relation, the familiar is in fact the familiar OF said patron but by their decree, you will have been given the rites to use them. Besides this, most ambitious spellcasters will utilize summoning and containment spells or rituals to enlist stronger entities into their service. This is both a double-edged sword AND a proving ground for those who are successfully able to put their mana where their mouth is can come away with a familiar stronger than most for combat usages, or for higher magical purposes such as long-ranged transportation. BUT if they fail to successfully restrain this summon or entity and the entity breaks out, then they've either failed and lost something VERY important or they've made an era-long enemy - most who attempt try for angels or demons and such that are extraplanar - thus living FAR longer than the summoner which, of course, will try multiple ways to get their justice done.
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For a basic contract circle, the spellcaster requires the Magic Circle spell alongside various incense matching the alignment of the creature to contract. There must be offerings or agreed payment bartered between the familiar and the spellcaster.