The album was mixed at Finnvox Studios, and mastered at Cutting Room Studios in Stockholm, Sweden. The album was also released as a limited edition noble book in 7" size (1000 copies), exclusively available via mailorder from Nuclear Blast records. The limited edition came with a differing track listing and bonus tracks "To create a warlike Feel" and "Out in the Fields".
Unia is darker and more aggressive than Sonata Arctica's earlier work, and has been described as a significant "turning point".
Vocalist Tony Kakko has once admitted that Unia being released just after Reckoning Night "was probably a shock to many people". He also described it as "a liberating experience. From there we took slowly some other direction". Commenting further on the album, he also stated:
Back then our band thought we are in some kind of pit and that we needed to do something different. To be honest, in that time it would have been better to do a solo album, but there was no time nor possibility for it. Unia is one of the closest albums to me personally.
Drummer and founding member Tommy Portimo said that Unia "brought many good things. I think our band needed that little shaking. [...] That album felt very natural. Although we realized is was very far from what we were before. But is just was a natural shaking."