Coming back to King Diamond, he left Brainstorm in 1978 to join another local band called Black Rose, but this time, as a vocalist. Originally, King was looking for another band to play guitar in, however, he saw an advertisement in the newspaper of this band who was looking for a singer. He tried to talk with them, saying he was a guitarist and a singer. He was lying as he wasn't a vocalist at all, but he asked if he could do both in the band, but they said that the guitarist prefered to play alone and that he was really good. King, who didn't have anything else to do at this moment, said he would give it a try anyway. It may have sounded a bit weird in the beginning as it was more screaming than singing when he tried to do high notes, but it started to sound better and better after some time, even though his voice wasn't very developed yet. At that time, they used to cover bands like Deep Purple and Rainbow, but they wrote some songs on their own too. At one point, a thing happened that made King and the guitarist go their own way, they found three new guys from another band: a bassist, a drummer and a keyboard player. When they joined forces, it was the first time they had a keyboard in the band, which really turned them into a direction where they started to write their own songs. They still did a couple of covers when they were doing live shows, mostly Deep Purple songs, which they considered as the greatest rock band around back then. They used to play more unusual songs like "Living Wreck", "Fools" and songs like that, but not "Smoke On The Water" and so on; they only played the harder stuff. To perform these songs, King had to hit a lot of Ian Gillan high notes. Once again, it was mostly screaming, but with practice it started to develop, and he began to hit the notes very clearly. One night, after one of their gigs, a guy came up and told King that he should use falsetto more, as he referred especially to the song "Road Ride". At first, King didn't even know what this word meant, but still he tried to use that voice more, and King forced himself to go higher and higher. King is purely self-taught, as he never wanted to take any vocal lessons; he considered he had no use to go in and rehearse scales or having some guy telling him how to breath instead of how he was breathing. He liked the way he did his vocals, and if he had to go in and breathe a different way, that would probably be destructive.

It's also in this band that the character of King Diamond as we know now really began to take shape, as Black Rose's biggest influence was Alice Cooper. For example, King was carried on-stage wearing a straight-jacket by two male nurses, and placed in a wheelchair. After the first song, King would throw off the straight-jacket, toss the wheelchair into the audience and get on with the rest of the show. They used various props, like to split open a pig's head before they went on stage and then filled up a doll with the entrails and blood. After the gig had starded, King would ritually sacrifice this doll in a satanic manner, again smearing himself in blood, and throwing the guts in the audience. He had made explode bombs which he made himself from stuff he stole from the lab he worked in developing medicine; he used some of those powders and stuff to make it, he would take a live torch and stick it into the holder to make it explode. It was very dangerous, but yet extremely effective. At the time, it was controversial enough as we could have seen them in the biggest Danish rock magazine with the headline saying "Black Rose: Violently Insane Musicians???", with a photo showing King in full make-up, holding a human skull, while being stabbed by the drummer. It was, and still is King's philosophy that when the audience pays to see you in concert, they expect more than just a perfect reproduction of the album they have; rather they come to see a show and a showman. Alice Cooper was one of the great showmen of his time and King aimed to emulate him; that's why an audience expect a visual and aural experience when they come to see them. At the opposite of what some may have thought, Mercyful Fate never played Black Rose songs at all, and nobody did ever heard the Black Rose material as King had the only copies of it that exists, or, at least, until the Black Rose rehearsal CD came out in February 2001.

However, King, which is a fully-educated and fully-trained chemist as he has all his diplomas, still used to work in a laboratory in Denmark. He was working on developing medicine to increase the blood flow to the kidneys. He had to test this stuff on three rats a day. Here is what he had to do in this job: first of all, he had to sedate these rats. Then, putting them on a wooden plate, with one thing under their teeth and their little legs were spread out and tied down. Then he had to open up the neck and insert little airpipes so it wouldn't choke. Then he had to find the main vein in the neck and isolate it to cut it. It was such a tiny work, with little scissors and stuff. He had to make sure that they had a little testing tube into the main vein where they could give it this medicine with a syringe. Once that was ready, then he had to go down to the front near the leg, go in there, and find his way all the way back to the kidney. He had to find the main vein going to the kidney, and then put a little magnet around that vein that would measure the blood flow to the kidney. They were hooked up to all these electronic machines and then whenever he finished one of these things, he'd get a seismograph type thing writing out all this stuff. When he finished one of these projects, he would give the rat a shot of air, which would simply give it a heart attack. It was under, so it would never feel anything, but still it was pretty gross to do. Once, he even assisted the main doctor operating on a dog, doing the same thing on it. Even though King thought it was a great job with secure income, he didn't have the time for both his job and music, so he had to make a choice, which wasn't easy for him to decide back then. He finally left his job to concentrate more on his music, but then he had to live on the social security for the two following years.

In November 1980, King left Black Rose after about 10 gigs. The whole thing started when the keyboardist showed up to the rehearsal one day and told the band that he sold his keyboard because he just got a new apartment with his girlfriend and so he needed the money to afford it. Then King got a phone call from Brats's guitarist Hank De Wank and his manager. They were looking for a new singer, and they knew of Black Rose and saw them live as well, so they approached King. King was not totally unknown in the entourage of the band as he used to hang out a lot at a record shop in the middle of Copenhagen. He knew one of the guys that ran the shop, Ken Anthony, who was also working with the guys of Brats, and suggested to King to meet the guys and to try out. So King then entered the band and gave it a try.

With King Diamond joining Brats, the band had an initial talk about what the band was gonna be about. As their style was half punk and half metal, King didn't want to sing punk and just wanted to sing the heavier stuff; that was a criteria for him to join the band. Hank and most of the guys were into that, so they started working together and writing extremely heavy metal songs. They didn't have demos of a lot of stuff back then, they did it for some songs, but many of them were never recorded on any tapes; they just remembered them as they've been rehearsing a lot, something like three or four times a week. It was also at this time that Hank De Wank changed his stage name to Hank Shermann. In January 1981, Brats invited a representative for CBS to come down to their rehearsal room in Copenhagen, to give him a listen to the material that was written for the upcoming second album. They had 9 songs ready, including "Death Kiss", "Combat Zone" and "Love Criminals". The new songs were definitely not what CBS had expected them to be. The next day, King and Hank were asked to come to the label's office. They were solemnly informed that the new material was far too heavy for them, and that if they wanted to cut another album it would have to consist of far softer material. King and Hank immediately refused this on behalf of the band. In addition, they were offered a ridiculously low budget to make a new album, so that led to the end of the band's relationship with CBS. In the two following months, the band rehearsed some new material and performed two more concerts; one in Denmark and one in Sweden, which is actually their first show outside of Denmark ever. As they were not really happy with what happened with CBS to drop the band, Yenz and Monroe left the band in March 1981. At the end of this same month, the three remaining members recorded a 3-track demo tape at The Rocktape Studio in Copenhagen. A good friend called Jan Musen helped out with the drum parts, as the bass parts were split between both Hank and Carsten. The songs they recorded were "Death Kiss", "You Asked For It" and "Running Away". The result was so much heavier than anything the band had ever done before that they decided that a change of the band's name was necessary. The name Back To Hell was considered for a while, but then the girlfriend of their manager suggested Merciful Fate. The name was chosen, but changed to Mercyful Fate when the classic gothic lettered logo was created, still in March 1981.