In addition to his planeswalker card, Karn has a creature card. A 4/4 that gets -4/+4 if he gets blocks or becomes blocked (Because he was a pacifist at the time, but eventually got over it). He can turn a non-creature artifact into a creature for 1 mana.
Karn has 3 planeswalker cards. True to his roots as an artifact creature, he is colorless. Karn was the first colorless planeswalker card. Karn shares the spot for most third most expensive (CMC wise) planeswalker with Garruk, Apex Predator. Still, the cost of 7 generic mana is deliberate; this is the exact amount of land produced by all three "Urzatron" lands combined. Turn-3 Karns are a selling point of Modern 'Tron decks, much to the rage of many other players.
Anyway, Karn's first 2 abilities are exiling stuff. Karn's first ability has a "cost" of giving you FOUR planeswalker loyalty counters. The only other planeswalker with a loyalty ability that gives more than 2 is Nicol Bolas. Karn's plus ability (cost aside) is not too exciting on its own. "Target player exiles a card from his or her hand." His next ability is exiling a permanent at a cost of -3. His final ability is unprecedented. He restarts the game except the non-aura permanents exiled by him start under your control. Enemy Emblems, Taken commander damage, Have poison counters? Poof, its all gone! Karn's planeswalker card is so weird that entire pages of the Comprehensive Rules are dedicated to this single card If you take a look at it's gatherer page, you'll see that it has 13 rulings on it .
His second incarnation as the Scion of Urza dials back on the unprecedented power of the first Karn walker and is much more suited to a support role. He's a 4 generic mana walker and starts with 5 loyalty. His +1 reveals the top two cards of your deck, and your opponent gets to choose which one to give to you. The other one gets exiled with a silver counter on it for later. This effectively gives you the worst of two draws, but hey, card draw is card draw. His -1 lets you nab any card from your exile zone that has a silver counter on it, just in case you really need that spell that got exiled earlier. His "Ultimate" is a -2 ability that plops down a little Construct artifact creature token that gets bigger the more artifacts you own. So yeah, VERY far from his Liberated form in terms of power, and is more in line with his pacifist nature. If your deck needs a little bit of extra support, and you don't mind getting the worst of two draws for a while, then he can be inserted into most decks without hassle.
His third incarnation, Karn the Great Creator, combines a little bit of both of his previous planeswalker abilities into one 4 generic mana Rare walker. With 5 starting loyalty, his static effect is in the form of a one-sided Null Rod, turning off all of the activated abilities of your opponent's artifacts. Sol Ring? Turned off! Sundial of the Infinite? Turned off! Ashnod's Altar? Turned off! Things get really weird if you happen to have Mycosynth Lattice out, which turns everything into an artifact. Because your opponent's lands are now artifacts, and mana abilities are also covered in the errata for Null Rod, you've effectively turned off your opponent's lands until Karn leaves the board, while your lands are safe and sound! This combo proved to be so oppressive, they had to ban Mycosynth Lattice out of Modern. His +1 animates non-creature artifacts with power and toughness equal to their CMC until the end of your next turn, letting you protect Karn for a turn. His -2 lets you tutor for any artifact card you own outside of the game or in exile and puts it into your hand. This makes it trivial to nab any win condition you want, so long as it's artifact based.
Perhaps the most amusing way to win a game of Magic (and arguably has a 100% win rate, technically speaking) is to Ultimate Karn Liberated with the Barren Glory as the only non-Aura permanent exiled by his loyalty abilities. Proceed to MULLIGAN TO ZERO, and then win on your upkeep. If you actually took the time to read Karn's 13 rulings, you would know that his Ultimate puts you on the play (meaning more than likely the opponent won't get to do anything 'the entire game').
|Planeswalkers of Magic: The Gathering|
|Original Five:|| Ajani Goldmane - Chandra Nalaar|
Garruk Wildspeaker - Jace Beleren - Liliana Vess
|Alara:||Elspeth Tirel - Nicol Bolas - Sarkhan Vol - Tezzeret|
|Zendikar:||Gideon Jura - Nissa Revane - Sorin Markov|
|Scars of Mirrodin:||Karn - Koth of the Hammer - Venser|
|Innistrad:||Tamiyo - Tibalt - Davriel Cane|
|Return to Ravnica:||Domri Rade - Ral Zarek - Vraska|
|Theros:||Ashiok - Kiora - Xenagos - Calix|
|Tarkir:||Ugin - Narset|
|Kaladesh:||Dovin Baan - Saheeli Rai|
|Other:||Dack Fayden - Vivien Reid - Kaya|
|Commander 2014:||Daretti - Freyalise - Nahiri - Ob Nixilis - Teferi|
|Pre-mending:||Bo Levar - Commodore Guff - Jaya Ballard - Urza|
|Planeswalker Groups:||The Gatewatch|