Dark Ascension is the second set in the Innistrad block and continues the story of Innistrad, a plane of gothic horror where humans fight a desperate battle against ghosts, werewolves, vampires, and zombies.
This set introduced the Undying mechanic which allows the creature with it to return from the graveyard with +1/+1 counters. Also new to this set was the Fateful Hour ability word which provides startling mechanics when life points are low, potentially turning the tide of an otherwise hopeless game.
Look for these popular Commander, Modern and Legacy, Vintage playable cards:
Mikaeus, the Unhallowed
Thalia, Guardian of Thraben
Sorin, Lord of Innistrad
Huntmaster of the Fells
And many more