You may have Dack's Duplicate enter the battlefield as a copy of any creature on the battlefield, except it gains haste and dethrone. (Whenever it attacks the player with the most life or tied for most life, put a +1/+1 counter on it.)
5/29/2014 Haste and dethrone are part of the copiable values of Dack’s Duplicate. If another creature enters the battlefield as or becomes a copy of Dack’s Duplicate, it will copy whatever Dack’s Duplicate is copying and have haste and dethrone.
5/29/2014 The ability of Dack’s Duplicate doesn’t target the creature.
5/29/2014 Dack’s Duplicate copies exactly what was printed on the original creature and nothing more (unless that creature is copying something else or is a token; see below), except it will have haste and dethrone. It doesn’t copy whether that creature is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.
5/29/2014 If the copied creature has in its mana cost (such as Grenzo, Dungeon Warden), X is considered to be 0.
5/29/2014 If the chosen creature is copying something else (for example, if the chosen creature is another Dack’s Duplicate), then Dack’s Duplicate enters the battlefield as whatever the chosen creature is copying. It will have haste and dethrone.
5/29/2014 If the chosen creature is a token, Dack’s Duplicate copies the original characteristics of that token as stated by the effect that put the token onto the battlefield. Dack’s Duplicate is not a token, even when copying one.
5/29/2014 Any enters-the-battlefield abilities of the copied creature will trigger when Dack’s Duplicate enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the chosen creature will also work.
5/29/2014 If Dack’s Duplicate somehow enters the battlefield at the same time as another creature, Dack’s Duplicate can’t become a copy of that creature. You may choose only a creature that’s already on the battlefield.
5/29/2014 You can choose to not copy anything. In that case, Dack’s Duplicate enters the battlefield as a 0/0 Shapeshifter creature, and will probably die almost immediately, when state-based actions are next performed.
5/29/2014 Dethrone doesn’t trigger if the creature attacks a planeswalker, even if its controller has the most life.
5/29/2014 Once dethrone triggers, it doesn’t matter what happens to the players’ life totals before the ability resolves. You’ll put a +1/+1 counter on the creature even if the defending player doesn’t have the most life as the ability resolves.
5/29/2014 The +1/+1 counter is put on the creature before blockers are declared.
5/29/2014 In a Two-Headed Giant game, dethrone will trigger if the creature attacks either player on the team with the most life or tied for the most life.