Everyone who's smart should switch the order of steps 2 and 3. Open your packs BEFORE completing the prologue. This will slightly increase your chances of getting multiple Death Knights since the free DK you get from the prologue can't be a duplicate.
Ignore this advice if you don't believe as I do that the Death Knight cards are on a higher average power level than the rest of the legendaries in the set.
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Just remember that minions can have negative health if done properly.