Blood

It takes multiple parts to create life. For the ruňans, there are four: physical body, breath (life), self and luuwa (nature). For the tīnonu and the sowūhua there are only the physical body and the soul where memories and any sense of self are.
In every case those parts need something tying it all together to create one whole creature. Blood is this uniting component, and as such has a few properties of its own. Though highly versatile and potentially a powerful ingredient in spells and alchemy alike, its use is also quite disputed among just about every culture in the world.
In the Sun Kingdom the use of blood as a part of any alchemical or herbal concoction is completely outlawed, with punishments ranging from a warning to being burned alive. In the regions where the Old Faith still holds strong, the use of blood is usually accepted so long as obtaining it doesn't break any of their other laws, in particular ones about harming and assaulting others.
But the liquid crimson doesn't have to come from humans. Animal blood is a popular alternative, and though not as effective, it does mostly the same job. In some recipes blood is simply used to heighten the effect and make it more potent, both in brews and spells (written or spoken). In other cases it is an integral ingredient, and depending on the specifics such as purpose and other ingredients animal blood may not be enough.

Some sowūhuan wisemen have a recipe for a poison combining the boiled juice of a yoth-cabbage and some other ingredients, and finally adding in the blood of the target. Once consumed, for example during a meal, the victim soon becomes very tired and goes to bed. While asleep, the poison slowly works to paralyze and eventually kill the victim.
In Ruňa, the effect of some spells can be increased with a sacrifice of the caster's blood. This works particularly well with divination, whether it is to find something lost or to see into the past or future. However, the caster might fall unconscious in the middle of the spell should too much blood be spilled. If so, the caster may find it impossible to return back to the realm of the living.