Hietala announced his departure from NIGHTWISH in January 2021, explaining in a statement that he hadn’t “been able to feel validated by this life for a quite a few years now.” He has since been replaced by session bassist Jukka Koskinen (WINTERSUN),who made his live debut with NIGHTWISH in May 2021 at the band’s two interactive experiences.

In an August 2022 interview with Finland’s Chaoszine, Hietala revealed that he went through a dark period in his life, which included depression, insomnia, anxiety and an eventual attention-deficit/hyperactivity disorder (ADHD) diagnosis. Speaking about how he eventually came to the realization that exiting NIGHTWISH was the right thing to do, Marko said: “It was a long process. Of course, the COVID year that was there, where I had a lot of time for soul searching, it obviously gave me the last incentive that I need something else, that if I just continue with this I’m just gonna get sicker and sicker. But, of course, it’s a process.

“I’ve been chronic depressive since 2010 or 2011, so I’ve been on a permanent medication ever since,” he revealed. “Sometimes you get used to the meds and you will need more. We did raise the dosage during the years also, but it just didn’t work. And now that I started to do… I had psychotherapy for over four years now, and then I also talked to psychiatrists and some doctors and did that also in Spain. Then my psychiatrist here in Finland said that I should do these ADHD neuropsychological tests, which I then did in Spain. And, okay, I got it.”

Hietala reiterated that he “had been thinking about” leaving NIGHTWISH “for a while” before making the final decision. “Because I had a lot of weight. And I tend to… With the attention disorder, it tells me that when there are lots of trouble, then the disorder makes it into a real chaos,” he explained. “There’s a shitload of stuff coming and going and no peace anywhere. And for a year or two, I was already waking up every night at three o’clock to bad dreams and anxiety. So I’d say that the whole process probably started already with my former divorce in 2016. That was a very sad time when you think about your kids and your broken homes and all that. And then, when I started to get clear from that, then there were, well, all kinds of things. I don’t really wanna go any deeper to what kind of things I’d gone through, but I’d gone through enough.”

Acknowledging that making NIGHTWISH‘s latest studio album, 2020’s “Human. :II: Nature.”, was a “difficult” experience for him, Marko denied that his mental state at the time resulted in a diminished role for him on the final LP. “I think the original idea was to have that… we’ll do a couple of solo vocal appearances, or one solo for me and Troy (Donockley), and the rest Floor (Jansen), and then the harmonies; that was the idea originally for that,” he said. “So I don’t know if it affected. I think it was sort of as planned. But at that time I already had serious trouble with concentrating and serious trouble with a constant black cloud over my head.”

In July 2022, Hietala told Finland’s Iltalehti that he had not kept in touch with NIGHTWISH since his departure or followed the activities of his former band.

In May 2021, NIGHTWISH keyboardist and main songwriter Tuomas Holopainen said that Hietala‘s decision to leave the band “came as a bit of a surprise.” He told Finland’s Kaaos TV: “Marko informed us in December of 2020 that he was leaving the band. And even though he has been very open about his state and problems during the past years, it still came as a bit of a surprise for us. So it was a really tough pill to swallow. And for a few days, I was actually quite confident that there’s no coming back, that this is it. I remember talking to Emppu (Vuorinen), the guitar player, and we were, like, ‘You think this is it?’ ‘Yeah, I think this is it.’ I mean, enough is enough. So much has happened in the past. Something that broke the camel’s back, as they say. Then, after some time had passed — a few days — we started to think that it’s been such a ride of 25 years, with so many ups also, that this is not the way to end it.”

Tuomas elaborated on NIGHTWISH‘s reasons for carrying on, saying: “I think we still have something to give, and that’s the main point. The music is still there. We felt that there’s still so much music that needs to come out from this band that, ‘Okay, let’s give it one more shot.’ And then finding the new bass player was really easy.”

He added: “It’s not like we do this just because we need to do it and there’s nothing else to do. On a personal level, I feel that there’s still so many stories and melodies that I want to share with the world with one lineup or another, so that’s why you want to continue and keep on going.

“I’ve said this a million times, that a lineup change is the ultimate energy vampire, and that’s how it really felt and still feels.”

In June 2021, Jansen spoke about Hietala‘s exit from the band in an episode of her “Storytime” YouTube video series. She said: “That was a very sudden surprise that, of course, was not fun at all. But we understand — I understand — it was a necessary thing for him to do. And from there, we had to think of how to continue without him, and that also, in preparations towards the virtual show, that was a huge challenge.”

She continued: “I’m very happy we found Jukka to play with us. And it was wonderful to get together, even though miss Marko, in April 2021 to start rehearsing for these virtual shows. Fortunately, we had a lot of time together, and it was very nice to go through the process of really not just knowing how to sing or play the songs but actually really feeling them, really communicating them, as you do with a show. We did that together. And then, of course, came the relatively unnatural setting of standing in a green bathroom (laughs) called the green screens all around us and cameras and people going for coffee instead of a real live audience. But that was really great. We had so much time that we could rehearse all of that and had a lot of fun doing that in the process.”

NIGHTWISH is currently working on the follow-up to “Human. :II: Nature.”, which was released in April 2020.

HIETALA released his last solo album ‘Pyre Of The Black Heart‘ in 2020, under Nuclear Blast Records.