SentrySeb was a rude, ignorant creature when he was a fawn. Then, he learned about sympathy. And went about crushing fawn's dreams. No, JK. This isn't about Sentry, it's about his mask and pelt. Antlers = Undecided, probably peacock.

Mask:

Ya know how over-used the skull mask is? Well, I decided to add to that bunch. If you can't beat 'em into a rock, join 'em, THEN beat 'em into a rock. So, Sentry is now an 'un-offical' ZOMBIIIEEEE! :3

History of his mask:

Sentry uses his mask as a way to tell everyone he's noble and brave, hiding under a rock. That's the skull, essentially. He doesn't want the skull to be his cage, but he is, alas, graced with terror to others. He isn't a kind soul, yet the mask can make him too kind, and eager to help. It goes against his will. Why he got it, no one really knows. He just woke up with that accursed piece of head-gear.

History of the Halloween set:

Well, how tricky this may be to answer...

Sentry wants to tell you himself. An emotional and gory thing. No, Jk again.

He 'acquired' it recently. Now, i say that because he hates it. He hates looking gory and over-used. He gets teased, but he enjoys the red, and appealing side of it. Or so he says. I think he's just too afraid of himself to admit he needs to take it off. Or he can't... hmm...