The masque contains the skill for Death to take a life, and the knowledge that the revelers are about to die.
A masque, or mask, is an artifact used to disguise oneself. The prince throws a masquerade, which is a ball in which everyone wears masques. The masques are symbolic of the fact that they are hiding behind their wealth and privilege, hoping to avoid the Red Death. Since Death embodied is an archetypal character, seen again and again in fiction, the masque is appropriate.
“It was a voluptuous scene, that masquerade.”
The masquerade has seven rooms, one for each of the seven deadly sins. As the partygoers celebrate, they are dooming themselves. When Death arrives, the partygoers all notice.
The figure was tall and gaunt, and shrouded from head to foot in the habiliments of the grave. The mask which concealed the visage was made so nearly to resemble the countenance of a stiffened corpse that the closest scrutiny must have had difficulty in detecting the cheat.
The revelers are annoyed that the visitor is wearing a masque made to look like a corpse. However, they soon realize that Death is not playing around. Clearly, he know what they do not- that they are about to die.
And now was acknowledged the presence of the Red Death. He had come like a thief in the night. And one by one dropped the revelers in the blood-bedewed halls of their revel, and died each in the despairing posture of his fall.
His masque demonstrates his knowledge that they will die, and the skills to kill them.