Claws of Cocytus or Infernal Prison should take out any assassin before it gets a chance to move. The spell will usually ignore chaff. So don't send your Lord of the Night to assassinate a commander in a Mictlan province full of Rain or Sun Priests.
Anyone scripted to Petrify could also be able to paralyze the Lord of the Night then kill it, maybe. Does Luck or Twist Fate have a chance to negate the paralysis from a failed petrify, or getting sucked away by Claws/Prison?
How do you get Lord of the Night anyway? Is it a nation-specific pretender?