Eye of Cthulhu

This is the moon god's servant. It flies over the world, ensuring that no one grows stronger than they should. If the eye sees this, it will attempt to destroy you. So that in the absence of its master, the world cannot resist him. The eye can travel between universes; its attention can be drawn to a bright light that illuminates the darkness of night. This light illuminates the terrarium, a very rare artifact. But the eye can only exist at night; it is weak in the sun, so it flees at the first rays of light. It will return only after it has healed its wounds, so don't let it do this and continue the fight the next night.

You're a hero determined to stop this "eye of terror." You're well prepared for battle, and by taking the terrarium, you activate it.
The eye of Cthulhu appears with a thirst to destroy you.