jgsugden
Legend
It is not an exception to the rule, but the spell has different language than most spells that makes it a little different:From PHB: Casting some spells requires particular objects, specified in parentheses in the component entry. A character can use a component pouch or a spellcasting focus (found in chapter 5, “Equipment”) in place of the components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell.
No cost is indicated for the blood. Nowhere in the spell does it say that it is an exception to the rule.
That language specifies that the blood has to be used in this fashion to create the circle. No blood, no circle.As part of casting the spell, you can form a circle on the ground with the blood used as a material component. The circle is large enough to encompass your space. While the spell lasts, the summoned demon can’t cross the circle or harm it, and it can’t target anyone within it. Using the material component in this manner consumes it when the spell ends.
Confirmed in Sage Advice: https://mobile.twitter.com/jeremyecrawford/status/935314398322626561?lang=en