Nemesis

A Nemesis is a special type of optional, respawning Crisis in Microcivilization.
A Nemesis is initially always invulnerable, meaning it cannot be brought beneath 50% health. Bringing it to 50% health before the crisis times out will give it the Damaged status, adding an extra 5 minutes to the respawn timer. When the crisis timer runs out, a separate timer will start in the lower-right corner. This timer will show how long it will take before the Nemesis respawns, what is required to render it vulnerable, and if you have rendered it vulnerable. 
Notes
* After unlocking Rare Technologies in the Ascension screen, these will show up at random points in the Tech Tree. A Nemesis is the last of three hurdles before the technology in question can be researched. 
* Currently available Nemeses are the Huns, Smallpox, and the Inquisition.
* A Nemesis can be skipped by using 5 Secrets.