Magic and Blood Sacrifice
Casting magic spells requires power, and this power must come from somewhere. For casting a few lower level spells the power can easily come from the Caster himself, or the Lifeforce of the world around him. to cast higher-level spells, or to power spells that last a long time, or to cast many spells, another source is needed.
A Magic-user may perform an animal sacrifice to gain power from the Lifeforce of the creature in order to power the spell. This can be done in two ways. In one, the caster may simply kill the creature and spill its blood before completing the spell. This method is relatively inefficient in terms of power transfer, but is safer when it comes to avoiding the corrupting effects of magic.
The second method is to complete an hour-long ritual, in advance of the need, where the caster makes contact with an entity of the Outerdark and sacrifices an animal to it, in exchange for a greater level of power and an increased chance of becoming corrupted by this exchange.
It has been noted by Magic-Users that it does not seem to matter with regard to the size of the creature so sacrificed. Similar results have been obtained via sacrifice of doves, songbirds, snakes, dogs and oxen. There are tales of particular circumstances or specific spells that have been more or less efficacious due to the selection of animal. For example, the Investiture ritual performed by certain cults to bind worshipers to the cause of their deity is said to require the sacrifice of a white bull (in the case of the Cult of Mayyarthu, god of forge and fire) or just specific parts of an adult goat (in the practice of Narapathenia, goddess of storms, dreams and destiny).
Blood sacrifice is the easiest and most powerful method of enabling spellcraft, but it is also the most dangerous, as corruption lurks with every stroke of the blade.