In order to understand Karn you have to understand his master and creator. Urza. Crazy jerk he was Urza. By the way The Brother's War which is the back story on the guy is one of the best books I ever read.
I related to Urza. I have a lot of the same personality faults. I use the artifact cycle as a guide on how to reign that shit in before I turn into a manipulative arrogant asshole whose friends have all but abandoned him for good reason.
Urza built Karn to kick the butt of the forces doing currently doing this to him.
Well Karn did kick their asses before. It was awesome and epic. Anyway back to the old old days.
First off even before the plot jumped off the guy was a little off. Urza never got people. Said people included his brother. Envy and self-righteousness blinded him to the fact that his on-going feud with the man was being used to fuel the machinations of Phyrexia, another world of what can only be described as mechanical demons.
I say feud, but it was more along the lines of a World War, again read The Brother's War. It is awesome.
As Urza created a mystical explosion and finally did in his brother it finally hits him. "Oh crap we were being manipulated by monsters from another world. Bummer"
His asshole tendencies turn into full on insanity after that. The mystical blast turns him into a near god immortal who spends the next couple millennia trying to figure out how to get his bro back. Did I mention he decides to kill on sight every phyrexian he sees.
(This bit is important though it doesn't sound like it right now)
Soon he becomes a major monkey in the wrench and they decide to send in less demon looking spies. The phyrexians were originally human but it's been so long they don't get the model exactly right, but they eventually make new and improved better spies. The main thing they got wrong in the original models were the bits down there. The higher ups in eviltopia decide to scrap the originals. As you can imagine one of them says screw that and flies the coop.
She (it gets ambiguous but I'm going with she here) meets up with Urza and they hang for a few centuries, him never realizing what she is. Eventually crap happens and she sacrifices her life for him. The way her biology works is that she has a stone that's tied to her life force. Normally the phyrexians would have it to keep her under control but she stole it back before she left. Before she dies she gives it to Urza.
Urza's still trying to figure out how to get his brother back and figures duh just go back in time. He figures out a way to make a time portal but living matter can't exactly make it through. He experiments to try to find something that can make the journey and discovers silver will do it. He then gets on making a silver sentient machine that will do what he wants. In order to do this he places the spy's stone into Karn.
By the way I should have mentioned it, people only put up with Urza because as much as they hate him he is a semi-omnipotent genius.
The Early Life
Lots of problems occur. First off Urza at this point is less crazy than he was but he still doesn't get people. Standard Frankenstein story here folks. He creates life and and doesn't get the philosophical ramifications of making a human being or something mentally close to it. When Karn starts acting a bit too human Urza starts having his memory wiped rather than deal with it.
Also at this point, Urza, in order to get the best assistants for his mad time experiments, founded a university. A guy who will eventually become Karn's friend starts bullying the crap out of him, calling him Arty Shovelhead.
Teferi was jerk as a kid. Though later they do save the multiverse together, and he's suffered enough burning alive for 40 years.
Another friend - these three will hang out a lot especially in the Time Spiral block - comes to his rescue and is the first person to treat Karn like well a person. In order to keep his sanity after the memory wipes he repeats to himself one phrase over and over again for centuries. "Jhoria is my friend"
Jhoria is the first person who questions Karn's status as just a lab tool, and starts getting him to question it as well. But as you can imagine with temporal experiments shit happens and everyone ends up with bigger problems than an existential crisis.
Remember how I said Teferi spent 40 years burning alive. Well lets just say there's a time explosion. It's all wibbly wobly and timmey whimmy and it causes about 10 thousand things to go wrong at once. Read Time Streams. Also depending on how you look at it, wibbly wobbly timmey whimmy, the events of Time Spiral are happening concurrently.
After the time explosion, Urza gives up on going back in time. He decides to fight the demon horde in the here and now. He does a bunch of stuff that would make the guys in Gattaca proud.
If it wasn't evident before let me say it now. Urza is a mad scientist. He creates several generations of heroes to fight off the Phyrexian horde, but this isn't about Urza. It's about Karn so I'm not going to go into how disturbing what he does is, but let's just say he genetically engineers a wife for his best friend to keep him under control without telling either of them. Did I mention the insanity.
Anyway Urza reassigns Karn to guard one of the heroes and a bunch of Urza's inventions the hero is suppose to use come what we in the fandom call the Invasion. The problem is well the memory wipes. Karn doesn't remember his past he just remembers his duty. I'm not at the invasion yet. In the process of protecting said hero Karn goes on a lot of cool adventures.
I also want to make it clear. Karn is sentient. He thinks and talks and has a personality. He's also generally a good guy.
Details get a little sketchy, but while guarding the pen-ultimate hero, he accidentally kills a guy. After that he becomes a strict pacifist. When he is captured by said hero's enemies they torture him but putting him into a moving room where he continuously falls on to dudes killing them.
Eventually the invasion comes. Karn discovers that the mad scientist had included him in his list of inventions the hero was suppose to use to repel the horde. Karn is suppose to be the brains of the hero's big giant airship.
All this comes to a head in a big giant explosion not unlike the one that killed Urza's bro. The explosion kills Urza, the hero, and the villians. It also fuses Urza's powers and it's hinted some of both the hero's and Urza's personalities into Karn.
Phenomenal Cosmic Power
Well Karn had the power of a near God. Karn's gone through a lot of changes but the one thing that stays consistent about the guy is his curiosity. He started life tabula rasa style. All he is he owes to experience and external influence. He does two things. The first is create a prob to check stuff out. It doesn't end well.
Karn didn't know how much juice that thing had. First it starts as just a trinket to who ever can get it, but then it starts mutating stuff.
The second is build a world. Mirridin. I mentioned he had near god like powers right. It's supposed to be Argentum after what Karn's made of ( latin for silver) but he left and well the steward went crazy and renamed it. Literally.
Said steward also locked Karn out of his own world. Which was only possible because his original one was cracking at the seams. Queue even more wibbly wobbly timmy whimmy stuff in Time Spiral.
Keep in mind said world went through time experiments interdimentional experiments, and two interdimentional overlays. Reality was bound to break. At the last second Karn gives up his near godhood to save reality, but realizes it was only that near godhood keeping that phyrexian heart of his from taking over what it should be obvious by now he has, his soul. In a last ditch effort to be a good guy he locked himself back in the world he created while he's being corrupted in order to contain it. He is becoming one of the demons he was designed to and did defeat. May Venser save him from this evil they both know before it's too late.