The question is Shadowborn Demon's 2nd ability. It checks my graveyard for 6+ creatures. If I don't have 6+ I have to sac one. What if the demon is my ONLY creature? Do I sac him?
Indeed you do. If Shadowborn Demon's ability exempted itself from being sacrificed, it would say something like "sacrifice a creature other than Shadowborn Demon." If the only way you can satisfy the instruction to sacrifice a creature is by sacrificing Shadowborn Demon, then you must do so.
This also means that you can sacrifice Shadowborn Demon to its own ability while you still control other creatures.
Yes, if the Shadowborn Demon is the only creature you control and you have fewer than six creature cards in your graveyard, you will have to sacrifice it. You are required to sacrifice a creature, and it is the only object you control that meets the definition of a creature.
This also means that you can sacrifice Shadowborn Demon to its own ability while you still control other creatures.
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