Hunters From Hyperspace
Assassins that mysteriously appear, attack someone, and then disappear after killing them.
The hero has severely injured and/or humiliated the Big Bad, and the Big Bad is considering sending assassins after him/her/it, as revenge. But the hero is behind a small army of allies/in an impenetrable fortress. So the Big Bad decides to send [B]teleporting[/B] assassins after the hero.
Hello, Unknown Troper. You'll need to get known to lend a hand here.