its seal-the redness and the horror of blood

[MEHRANQ]

I've checked Every Dictionary and I couldn't find any meaning that fit with this context:

Novel Name: The Masque of Red Death
Arthur: Edgar Allan Poe

The “Red Death” had long devastated the country. No pestilence had ever been so fatal, or so hideous. Blood was its Avatar and its seal—the redness and the horror of blood. There were sharp pains, and sudden dizziness, and then profuse bleeding at the pores, with dissolution.

 

[Tarheel]

its seal - its mark, its defining characteristic

 

[jutfrank]

The word also carries the sense of fate, I think. The Red Death had sealed the fate of many.