- You may also be looking for the cartoon episode by this name.
Comic book version
He was simply referred to as Smurf (and sometimes identified as Pretentious Smurf), who in Papa Smurf's absence competed against Brainy Smurf for leadership of the village. Through vigorous campaigning Smurf had successfully led the entire village to outvote Brainy Smurf by an overwhelming majority. To celebrate his victory, Smurf had put on a gold Smurf hat and pants and had all the other Smurfs call him King Smurf, which only elicited laughs until he had appointed Harmony to be his official herald and Hefty as his captain of the guard. He had Hefty force the other Smurfs to build him a castle that he could live in. However, only Brainy, his former campaign opponent, heaped praise unto King Smurf for his magnificence as he showed up in the new palace in full King Smurf regalia, wearing a crown and a red cape with furry white trim and carrying a scepter with a mushroom top.
When Jokey played his usual "surprise" giftbox joke on Hefty, he took Jokey to court. There the King took the whole matter in good stride, saying he has a sense of humor, and let him off. Unfortunately, Jokey gave an exploding gift to the king, and after it exploded in his face, he put Jokey in jail. This sparked a rebellion in which a group of Smurfs broke Jokey out of jail and fled to the forest, though it led to Brainy's capture. Realizing that he now had a rebellion on his hands, King Smurf had forbidden travel into the forest and even sent an army of loyal followers to follow him into the forest to find their secret hideout. However, a good deal of them only joined King Smurf's army just to secretly join the rebels.
With more Smurfs joining the rebels' ranks, King Smurf had a high fence erected and a moat surrounding the village so that the rebels could not re-enter and also that no one inside the village could join the rebels. Nonetheless, during a charge attack by the rebels, a gap was formed in the fence that enabled the rebels to enter the fortified village. During the fight inside the village, one Smurf used an explosive to blow up King Smurf's castle, which forced him into a desperate situation of being surrounded by only his loyal guards.
However, the fight soon ended when Papa Smurf returned, chastising the Smurfs for "acting like human beings" and wrecking the entire village in the process. Realizing the error of his ways, King Smurf relinquished his role in order to help fix the village with his fellow Smurfs joining him, and his King Smurf clothes were used as part of a scarecrow in their fields.
Cartoon show version
He was Brainy Smurf, who in Papa Smurf's absence decided to make himself king so that his fellow Smurfs would follow his orders without questioning them. As King Smurf, Brainy appointed Harmony to be his official herald and Hefty as his captain of the guard. He had Hefty force the other Smurfs to build him a castle that he could live in, with only Harmony to give him praise for his magnificence. Brainy's story from that point on played out in similar fashion to the comic book version of King Smurf, only without the campaign to search the forest for the rebels' hideout, and also when Papa Smurf returned, he chastised all the Smurfs for ignoring the danger of the village dam about to burst. Immediately they dropped what they were doing and allowed the overflowing water to be released from the dam, destroying Brainy's royal palace in the process. As did his comic book counterpart, Brainy realized the error of his ways and relinquished his role as King Smurf to help fix the village with his fellow Smurfs joining him.