Custom Card Ideas: Poser
Intro
actually was inspired for this card from another source, so yes, thus far this is my only non-original card (aside from the Blue Turtle Shell, of course). In practice, in turns you into a monster to attack another Munchkin. However, I didn't feel the stakes (or 'logic') of the card was the way I wanted it, so I re-created the card.

Previous Version
This was the first incarnation of the card and its description (you can see said card below): "Treat your character as a wandering monster, adding their combat bonuses against another player currently in combat. if the munchkin currently fighting the monsters wins, the munchkin takes 3 items of their choice from the Poser and gains a level. If the Poser wins they get a level for every munchkin in the combat which can be a winning level. The Bad Stuff: The Munchkin dies." A bit wordy, I know, but it's a card that temporarily bends the rules to allow direct player vs. player combat.
It was popular and good, but I soon discovered that the Poser got the worse end of the deal every time. A Munchkin can banish (or kill) the monsters with a simple card's effect, rendering Poser wasted. Also, a Poser was putting 3 of their Treasures on the line for just a level, which simply didn't make sense to me. No one would play the Poser unless they were positive they would get out Scot-free.
Current Version
So, I re-worked the card (picture above): "Treat your character as a wandering monster with your combat level as your level. However, if the monsters lose, you die and the munchkins gain a level and 3 of your Treasures before Death. If you win, your Bad Stuff is Death and you get 3 Treasures from them before Death. you also gain a level for every munchkin in combat which can be the winning level." This way, both sides have 3 Treasures and a Level to gain (for each Munchkin) possibly allowing you to jump from Level 8 to win! Even better, if you are a Dark Poser (using the Race Enhancer Dark from the core Fantasy deck) you can win from Level 6, even if the Munchkins get away!
Strategy
While the user of the Poser card still to put a bit of himself on the line, he's not alone. He can be bolstered by Monster-only items and Monster Enhancers, as well as be joined by other Monsters, say a Level 20 Plutonium Dragon or similar things. And, if you enjoy adding a little bit of supplementary rules (which we do from time to time) if you play this card with a Mojo (from the Munchkin Zombies deck) or the Vampire race (from Munchkin Bites) you could be considered an Undead monster, allowing other Undead monsters to be played with you for free! Or with the combination of the Tentacle item (from Munchkin Cthulu) and the Dungeon of Manga Wrangling (from the core Fantasy deck) you can get a +5 against Female players, or any number of other combinations of the decks. Remember, this card is to make a Monster out of a Munchkin; the rules can and will bend a little.
The most possible safe way to play this is actually the least eventful. Find another, less fortunate Munchkin player than you, then play the Poser against them. For guaranteed results, make sure they can't run away from you, allowing you, without fear, to take up their stuff and kill them for a level. It's a jerk move that may cause people to hate you, so wait for the end of the game to play it.
The best, cinematic use of this card is against that Level 9 Munchkin going for the win. Playing this card smartly will set you up to win as well, and then everyone else in the game realizes they have to chose which side they'd rather help win, because at this point, it has to be one of you (unless someone plays a card to end combat).
Deck Compatability
As per usual, this is a very generic card that can be added to most any deck. In the pursuit of self-interest, it's best when used when plenty of Wandering Monster cards may be played. Against cards like the Wizard which can use a skill and remove all Monsters from the fight, you'll want to be careful so not to waste this card.
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More Munchkin
I've got plenty more Munchkin Custom Card Ideas! Also, as I've spent far too money on numerous Munchkin decks and expansions, I can tell you which ones are worth it, which aren't, and why.