Success is measured in blood - yours or the enemies.
— Imperial Proverb, Warhammer 40,000
"Only flesh with its soul - its blood - you must not eat."
"The blood is the life."
Xander: Why blood? Why Dawn's blood? I mean, why couldn't it be, like, a lymph ritual?
Spike: 'Cause it's always gotta be blood.
Xander: We're not actually discussing dinner right now -
Spike: Blood is life, lackbrain. Why do you think we eat it? It's what keeps you going, makes you warm, makes you hard, makes you other than dead. 'Course it's her blood.