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Top two were Boros at my prerelease.

It might be because Boros is the guild that benefits most from the guild pack (then it has a critical mass of battalion creatures). In a draft or a regular sealed when they're more scarce it might not dominate so hard.

Played in two pre-releases Orzhov both times.
In the first one (Friday midnight)I got only 5 extort cards total, but a ton of removal, but splashed blue and managed to win 2 of the 3 rounds (lost in round 1 to Boros player who got all the good Boros commons and uncommons, won against another Orzhov deck that splashed blue and a Dimir deck that splashed black).
The second one I did today (Saturday 2:00pm) got a crazy pull: Ghost Dad out the of the guild pack, Deathpact Angel, and Alms Beast out of two random boosters plus three of the guild mages. Won round 1 against another Orzhov deck (new player ran 60 cards in his deck, even though the store announced 3 times that the minnimum was 40 cards most of the new players to pre-release still build 60 card decks), won round 2 against Dimir deck that has a lot of good mill, but too few creatures, and won final round against Simic with splash of red that got too few creatures the first gaem and no blue lands the second game.

At my 40 man prerelease, Boros took 1st with an undefeated record. The next 4 players all tieds, which contained 1 Dimir (ME!!!), 2Orzhov, and 1 Jank(I don't know how this guy was winning with 4 colors :/)

My opinion on top cards: Holy Mantle this card wrecked me. Bad. Crypt Ghast was fantastic ramp and the Extort saved my butt a few times even though I was Dimir. Dimir's Promo is the most deadly promo out there. Boros has a lot of removal and psuedo-removal it seems.

Gruul was terrible for me. I destroyed them so quick. Orzhov seemed to do the worst except for one guy who got really good cards from his packs, and I take back what I said about Cipher. Although some cards are still terrible like Last Thoughts alot of them are amazing. The threat they pose forced Boros players to change tactics, Gruul players to wait on Bloodrushing, and Orzhov suddenly had to race.

Simic seems the weakest in my experience.

A friend and I just got done with a 2HG sealed prerelease event, and we placed 6th out of 30 pairs, having only lost once. I ran Orzhov with only extort and removal, and he ran Gruul with some white for Aurelia and a couple other tricks. It worked really well since most of my extorters were decent on defense and he had some big-backed reach guys to defend us while he built, and the opposition could never get a foothold on the battlefield because I removed just about everything that seemed problematic. Extort was great since it hits both opponents, for a total loss of two and gain of two, for just one extra mana. With three extort guys out, I could cast Smite and kill a creature and cause 6 life loss to their total and gain six for us. It was just nasty. Our one loss was basically to a mirror match, the only differences being a few rares and the like. We did great, got packs, and most importantly, had a great time.

Lesson to be learned: Orzhov and Gruul for 2HG, so neither player is trying to take the colors of the other deck and you can make solid, consistant decks....and then Extort for the win.

42-person event
1. Dimir (insane pulls)
2. Orzhov
3. Boros
^These were all undefeated.
4. Dimir (splash white for Gideon and Deathpact)
5.Orzhov

8-man event
1. Boros
2. Boros (me)

45-man event
1. Orzhov
2. Boros
3. Gruul

In the first one, I placed 9th with 3-1, my only loss to fourth. I played heavy Extort Orzhov.