Blood is the source, blood is power, blood is life
Blood is the essence of vampires. Vampires are born in blood. Vampires feed on blood.
When the Master rises in 'Welcome to the Hellmouth', he rises from a pool of blood. Blood is the key to his freedom and he sends several of his minions out to complete the Harvest - which involves killing lots of people by drinking their blood - allowing the master ti build up strength and break out of his dimensional prison.
The Sleeper will awaken. The sleeper will awaken. And the world will bleed.
Luke, Welcome to the Hellmouth
My blood runs with yours. My soul is your province
The Master, The Harvest
Still trapped underground, the Master finally breaks free when Buffy confronts him. He easily bites her and drinks her blood, giving him the strength to get out.
In 'When She Was Bed', several of the Masters followers plan to bring him back. To do this they must complete an ancient ritual. The blood of those that witnessed his downfall must be drained over his squeleton.
In Sunnydale to cure Drusilla, Spike eventually finds out how to bring his beloved back to health. They must perform a ritual in a church, in which Angel, Drusilla's sire must bleed to death.
From the blood of the sire she is risen. From the blood of the sire shall she rise again.
Spike, What's my Line.
Demons & Rituals
The frat boys that worship Machida have several rituals including blood.
In blood I was baptised, in blood I shall reign, in his name.
Richard, Reptile Boy
In 'Becoming' Angelus awakens ancient apocalyptical Acathla. Once again the ritual involves blood.
And to close the portal, Buffy must also shed Angel's blood. She drives a sword through him, plunging him in to hell and shutting the vortex created by Acathla.
I will drink... the blood will wash in me, over me, and I will be cleansed. I will be worthy to free Acathla. Bear witness... as I ascend... as I become.
Angelus, Becoming I
Blood. Of course. The blood on my hands must be my own. I am the key that will open up the door. My blood. My life.
more to come soon