(to the tune of "12 days of Christmas") 
 
 
On the first day of X-Risk I suddenly could see:
The end of humanity
On the second day of X-Risk I suddenly could see
 Nuclear War!
And the end of humanity
On the third day of X-Risk I suddenly could see
 Pandemic Plagues
 Nuclear War
And the end of humanity
On the fourth day of X-Risk I suddenly could see
 Bioengineering
 Pandemic Plagues
 Nuclear War
And the end of humanity
On the fifth day of X-Risk I suddenly could see
 Unfriendly AI...
 Bioengineering
 Pandemic Plagues
 Nuclear War
And the end of humanity
On the sixth day of X-Risk I suddenly could see
 One nanite making two nanites making...
 Unfriendly AI...
 Bioengineering
 Pandemic Plagues
 Nuclear War
And the end of humanity