Todesengel (Civ4 FFH)
This unit is nice for sneaking around and picking off militarily weak units in a stack, or picking off lone units. It is not good as a primary city attacker or defender. Compared to the Shadow that it replaces, it has the Taskmaster promotion.
Angels of Death are invisible and start with the Reaper promotion, decreasing the population of cities they attack by 1.
"One may not escape the angel of death, nor say to him, 'Wait until I put my affairs in order,' or 'There is my son, my slave: take him in my stead.'" Where the angel of death appears there is no remedy.
The courtesan is invisible to most units, and attacks the weakest unit in a stack, making it the perfect counter to powerful Summoners or Archmages.