By consuming 1 pint of blood from a spellcaster killed within the last 24 hours, you can attempt to learn a spell that spellcaster knew. Select one spell available to the dead spellcaster (this must be a spell on your spell list); you gain the Knowledge of this spell for 24 hours. During this time, you may write it down (or teach it to your familiar, if you are a witch) using the normal rules for copying a spell from another source. Once you have learned it, you may prepare the spell normally.