Wicked K was the 'prize' one receives for opening every Trickster Vault in the game. Once all the vaults are unlocked a door opens behind Vulgrim in his store that allows entry into a Arena like stage where the only opponent is Wicked Killington. Not that he needs much him proving himself a skilled combatant with a high amount of health fond of constantly pelting the Horsemen with powerful attacks and bad jokes. He drops one of the most valuable Creature Cores in the entire game.
During her journey to face the Seven Deadly Sins the Black Rider Fury participated in The Crucible and following her defeating all of it's waves in one go she faced off against it's master: none other than Wicked Killington himself. Upon his defeat he leapt onto his hat and asked: "Do you hear the fat lady singing?" before flying away. Vulgrim latter shows shock at Killington losing to a Horseman and Fury asks what Killington's story is only for Vulgrim to state that his secrets are larger and more sinister than most.
On Death's journey to resurrect humanity, the Pale Rider participated in the Crucible, a supernatural arena located in its own dimension. Unlocking the different segments of the arena, Death eventually conquered all one hundred of its waves in one go, prompting the arena's caretaker to direct the Pale Rider to his master, who waited at the top of the tower located in the middle of the arena.
Upon reaching the top, Death discovered the Master of the Crucible to be none other than Wicked K. The two battle one another, and upon K's defeat, the Wicked departed Death atop of his top-hat, telling Death to remember him as a lover and not a fighter, before screaming into the distance yelling the words "Tea time!"
During his adventures through the ravaged Third Kingdom, War stumbled upon K in several secluded areas, who swiftly challenges him to a duel. War bested him, but several more times did they meet again, with each fight fiercer than the last. But K relinquished a healthy bounty of souls to his opponent whenever he was defeated, keeping up a friendly demeanor the whole time.
K could be described as a well-spoken but insane English gentleman. He speaks with a refined vocabulary and is generally very polite towards the people he meets, treating them as well-respected friends and colleagues, even if they're battling. He might also believe in fair play, if his note about "no low blows" is anything to go by.
Even in combat, K is an elegant and respectful individual, using pinpoint attacks instead of large, smashing ones and slowly walking rather than running. He even bows his hat when he knows he's been bested.
While he has demonstrated himself to be a formidable fighter, the full extent of K's powers have yet to be seen. Regardless, what is known about K is that he uses only two pieces of equipment on the battlefield: his walking cane and his top hat. He wields his cane in the same way a fencer would use a sword, stabbing and slashing at lightning speeds. His hat is a more interesting subject; it acts as both a weapon and a means of transport. K can throw his hat, using its sharp edge to cut his enemies, enlarge it to block attacks, or even use it to teleport, be the distance vast or short. His hat is even capable of flight, as he has demonstrated when he flew away from Death after they battled at Crucible.
According to Vigil Games' general manager, David Adams, Wicked K was a character that the designers had in mind for two years. He is a caricature of Vigil's British contingent and was not added to the game until the final month of programming as an in-game joke.
K sometimes mentions the Justice Lords, but who they are in the Darksiders Universe, if they exist at all, is unknown. It could also be a reference to the DC Comics superheroes the Justice League.
The K could stand for Killington, as that is the filename used for his textures.
Wicked K was intended to be a reference to Buzz Killington, a recurring character on the animated TV series Family Guy and a favorite in-joke of the Vigil staff during the development of the original Darksiders. The designers had actually wanted his in-game name to be Wicked Killington, but THQ's legal department was concerned about intellectual property issues and asked that it be changed, so it was shortened to Wicked K.
In the Darksiders III DLC The Crucible he is finally referred to as Wicked Killington indicating a change in intellectual property concerns (regardless of whether or not the old concerns are still valid).