"Blood was its Avatar and its seal..."
See in text (The Masque of the Red Death)
The noun “Avatar” refers to the physical manifestation of something, often an idea or deity. The Red Death manifests in and uses the blood to “seal” itself to the body. The result is that blood, despite being natural and vital to life, becomes a source of fear. This description serves to estrange the physical body by casting it as an uncontrollable source of anxiety rather than a comfortable constant, enhancing the horror elements of the story.