So, I'll keep it short.
Embreached
Unmemorable and completely pointless, if you ask me. Their music seemed like a mix of metalcore and melodeath, and it didn't interest me. What was funny that as soon as they started playing, a pit formed, and this pit kept its place through the whole gig until Slayer finished.
Amon Amarth
One of the two reasons why I came to the gig. They did a great job with the amount of time that was given to them (half an hour), and their setlist was well chosen for this.
Death in Fire
Asator
Twilight of the Thunder God
Runes to My Memory
Free Will Sacrifice
Call of the Black Birds
The Pursuit of Vikings
I saw them this summer at Tuska and they did their job as well as then; it was interesting to see them in two very different venues. The only problem was that the audience wasn't so enthusiastic (well, they were there for Slayer...). I was at the front and I had to be actively creating pits to get any proper response from the audience. There was the bigger pit behind me but I couldn't really see what the going was like there.
Mastodon
I went for a beer with my friend and we only saw the last song. I won't say anything, as I didn't hear them long enough for them to make an impression one way or the other.
Trivium
I saw about half of their set. I was expecting them to be total shit since I've seen a few other metalcore bands live before (A7X and KSE) and they all sucked. However, Trivium pleasantly surprised me; while they were metalcorish, they weren't that bad and I might have enjoyed them if I had been closer to the front.
Slayer
Well, most people were obviously there for Slayer, no questions asked. I'm a bit annoyed, however, that they had Trivium and Mastodon as openers, as that just didn't fit in with their style. I heard some people talking about how they had only come for Mastodon/Trivium and not Slayer. This diluted the audience down a bit, since not everyone was a Slayer fan.
Nonetheless, it was a great gig in all respects but the audience (which was still better than you'd find at almost any other gig). They had absolutely stunning lights, complete with a cool LED screen behind the stage showing all kinds of Slayery stuff (upside down crosses during South of Heaven and fighters during War Ensemble for example). They kept this hidden very well; the gig began with a white curtain in front of the stage, and then the intro riff for Flesh Storm began and the whole thing dropped, revealing the awesome lights. The audio quality was a bit muffled sometimes, but that's Jäähalli for you. It seems there are always sound problems there.
The set list was excellent, including the entirety of Reign in Blood:
1. Flesh Storm
2. War Ensemble
3. Chemical Warfare
4. Ghosts of War
5. Jihad
6. Psychopathy Red (new song)
7. Seasons in the Abyss
8. Dittohead
9. Live Undead
10. Cult
11. Disciple
12. South of Heaven
13. Angel of Death
14. Piece by Piece
15. Necrophobic
16. Altar of Sacrifice
17. Jesus Saves
18. Criminally Insane
19. Reborn
20. Epidemic
21. Postmortem
22. Raining Blood
Highlights were South of Heaven, Psychopathy Red, Cult, Disciple, Angel of Death, Raining Blood, and Jesus Saves.
There was a big circle pit about 10 meters from the stage; it was pretty weird since there were people standing in the middle of the pit without doing anything, and everyone was moshing around them. I don't know if there were more pits; probably, but not as many as in Tuska.
It was an awesome gig, definitely one of the best I've been to. Slayer, while it was great to see them at Tuska, definitely deserve their own gig. It just makes life damn hard for the openers, though.