Feeding

Vampires must drink blood to survive.  As you explore Nosgoth and gain experience as a vampire, your capacity to hold blood increases.  Blood is consumed slowly to maintain your existance.  It's also lost when you take damage.
How To Feed
1Attack enemies.  One hit before enemies die, they sway about the screen in a Waver state.
2Approach an enemy while it's in a Waver state, and press O to feed.
Note ~You can also feed by using the Blood Gout and Blood Shower spells or by equipping Flesh Armor.
Blood Types
The four types of blood each have a different effect:
Red
Blood
Increases Kain's Blood Vial (Health Meter).
Black
Blood
Decreases Kain's Blood Vial (Health Meter).
Blue
Blood
Ghost/magic blood.  Drinking blue blood increases Kain's Magic Meter.
Green
Blood
Poison.  If Kain absorbs any green blood, his Blood Vial turns green and blood loss accelerates until Kain heals himself with Anti-Toxin.
Note ~ Green blood wears off in time, if Kain has enough blood in his Blood Vial to wait it out.