Master Warcraft — ruling 1

If a turn has multiple combat phases, this spell can only be cast before the beginning of the Declare Attackers Step of the first combat phase in that turn.

Master Warcraft — ruling 2

If the defending player controls a planeswalker, the person who cast Master Warcraft first chooses the complete group of creatures that are going to attack. Then, for each of those creatures, the active player chooses who or what it's going to attack.

Master Warcraft — ruling 3

You can decide that a creature won't block.

Master Warcraft — ruling 4

You choose attackers and make blocking assignments regardless of whether it's your turn and regardless of whether the creatures are attacking you. Your choices must be legal within the normal rules for attacking and blocking.

Master Warcraft : règles, fonctionnement et interactions (MTG)

Master Warcraft 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

Cast this spell only before attackers are declared. You choose which creatures attack this turn. You choose which creatures block this turn and how those creatures block.

How it works : Master Warcraft

Original English text — Wizards of the Coast / Scryfall

#1September 20, 2013· Wizards of the Coast
If a turn has multiple combat phases, this spell can only be cast before the beginning of the Declare Attackers Step of the first combat phase in that turn.
No other card is named in this ruling.
#2April 1, 2008· Wizards of the Coast
If the defending player controls a planeswalker, the person who cast Master Warcraft first chooses the complete group of creatures that are going to attack. Then, for each of those creatures, the active player chooses who or what it's going to attack.
No other card is named in this ruling.
#3January 1, 2006· Wizards of the Coast
You can decide that a creature won't block.
No other card is named in this ruling.
#4October 1, 2005· Wizards of the Coast
You choose attackers and make blocking assignments regardless of whether it's your turn and regardless of whether the creatures are attacking you. Your choices must be legal within the normal rules for attacking and blocking.
No other card is named in this ruling.