46 - Power Word: Kill. Boy, was the paladin shocked.
47 - "You see five hundred goblins coming towards you." Oh, wait, the party survived without a scratch and only lost two henchmen. Forget that one.
48 - Kobolds with plasma rifles. (What? Why should PCs be the first to find the crashed spaceship?)
49 - Great red wyrm in its volcanic lair faces adventurers. First round: Snatch as many as possible. Second round: Dive into the lava, bullrushing anyone who isn't snatched. Proceed to hold breath for a couple of rounds until the fighter melts, then surface somewhere else and start over.
50 - Formians pour out of a gate, chanting 'Resistance is futile. You will be assimilated.' They dominate every villain the PCs are facing and add them to their forces. They start constructing castles in the shape of huge cubes a mile on each side. They assimilate entire cities, nations and continents into their empire. And they ignore the PCs until the heroes attack any formian, at which point they throw legions at the PCs and do other unfriendly things.
For bonus points, introduce psionic items in the form of insectoid parts that must be grafted to your body to work. Then give every assimilated villain/NPC some of these to wreak havoc.