Guess what? This Trade Server spawns the:

That’s right, the Monoculus! Also known as the Eye Boss. Added in the last TF2 Halloween update of 2011.

Each 12 minutes, the Monoculus will spawn alongside with it’s portal to get there. You might say, portal?

That portal. Once it spawns, go check the bridge outside of the Stock Market, but do it quickly!
But beware! The portal only stays open for a limited time. Once you’ve entered the portal, you’ll be
teleported to an arena. Specially made for the Monoculus.