Blood Magic

Blood Magic is a forbidden and outlawed practice of magic in the modern sorcerer world but one that can come in handy in certain life or death situations. Blood magic is a powerful form of magic used to cast very advanced spells, but it requires the caster to actually sacrifice a certain amount of their own or another's blood in order to cast it. Usually this is done by making a cut or gash in the casters hand and pulling a mass of blood out with their own magic. The blood then serves as the catalyst for a powerful spell. This practice is outlawed however as not only does it come at great physical cost, but it can produce various side effects on the caster. For one thing if a sorcerer exhausts too much blood they will have very weakened power or even out right lose their ability to use magic until their body regenerates enough blood cells. Other side effects that can occur as a result of overuse of Blood magic include sever health issues, or even corruption and destabilization of one's own mana resulting in the caster going mad or dying themselves. Most Sorcerers who perform a blood spell will show signs such as discoloration of their eyes for a period of time but if the magic goes too far it can cause their eyes to permanently turn a deep crimson red indicating their soul has become irreparably corrupted. 
Despite its drawbacks Blood Magic can have its uses and practical application in certain situations. One blood spell can be used to cheat the system and allow a sorcerer to use their bloodline's master spell which can come in handy in certain life and death situations but still comes at a cost to the caster. There are also blood spells that can be combined with constructs to make them appear as real as any other flesh and blood creature, but this still will only last a short time and thus is not an effective spell to use unless genuinely in need. There are also methods of blood magic where more powerful and complex spells require a full bodies worth of blood. This would normally require a blood sacrifice of a living person to cast it quickly but the more patient minded can actually take the blood little by little. A liter here and a liter there like giving blood donations until the full amount is collected and pooled. However, this can have several side effects. Not only would this take a minimum of a week to gather the necessary blood without harming the donor, but it will also leave a sorcerer nearly powerless as the removal of so much blood strips them of their magic until their body can fully recover and regenerate on its own.