Dead Reckoning — ruling 1

You must choose two targets as you cast Dead Reckoning: a creature card in your graveyard and a creature on the battlefield. If you can’t, then you can’t cast the spell.

Dead Reckoning — ruling 2

If the targeted creature card is an illegal target as Dead Reckoning resolves (because it’s no longer in your graveyard, perhaps), you can’t choose to put it on top of your library. No damage will be dealt.

Dead Reckoning — ruling 3

If the targeted creature is an illegal target as Dead Reckoning resolves (because it’s no longer on the battlefield, perhaps), you may still put the targeted creature card from your graveyard on top of your library. No damage will be dealt.

Dead Reckoning — ruling 4

Dead Reckoning deals damage equal to the power the creature card had while it was in your graveyard. This could matter if that card is something like Lord of Extinction or Tarmogoyf whose power is determined by a characteristic-defining ability that checks the contents of your graveyard.

Dead Reckoning : règles, fonctionnement et interactions (MTG)

Dead Reckoning is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

You may put target creature card from your graveyard on top of your library. If you do, Dead Reckoning deals damage equal to that card's power to target creature.

How it works : Dead Reckoning

Original English text — Wizards of the Coast / Scryfall

#1March 1, 2010· Wizards of the Coast
You must choose two targets as you cast Dead Reckoning: a creature card in your graveyard and a creature on the battlefield. If you can’t, then you can’t cast the spell.
No other card is named in this ruling.
#2March 1, 2010· Wizards of the Coast
If the targeted creature card is an illegal target as Dead Reckoning resolves (because it’s no longer in your graveyard, perhaps), you can’t choose to put it on top of your library. No damage will be dealt.
No other card is named in this ruling.
#3March 1, 2010· Wizards of the Coast
If the targeted creature is an illegal target as Dead Reckoning resolves (because it’s no longer on the battlefield, perhaps), you may still put the targeted creature card from your graveyard on top of your library. No damage will be dealt.
No other card is named in this ruling.
#4March 1, 2010· Wizards of the Coast
Dead Reckoning deals damage equal to the power the creature card had while it was in your graveyard. This could matter if that card is something like Lord of Extinction or Tarmogoyf whose power is determined by a characteristic-defining ability that checks the contents of your graveyard.
No other card is named in this ruling.