Horde Ambusher — ruling 1

Turning Horde Ambusher face up after blockers have been declared won't remove any creature from combat or change how any creature is blocking.

Horde Ambusher — ruling 2

If a face-down Horde Ambusher blocks and is then turned face up, its first ability won't trigger because it didn't have that ability when it blocked.

Horde Ambusher — ruling 3

Morph lets you cast a card face down by paying {3}, and lets you turn the face-down permanent face up any time you have priority by paying its morph cost.

Horde Ambusher — ruling 4

The face-down spell has no mana cost and has a mana value of 0. When you cast a face-down spell, put it on the stack face down so no other player knows what it is, and pay {3}. This is an alternative cost.

Horde Ambusher — ruling 5

When the spell resolves, it enters the battlefield as a 2/2 creature with no name, mana cost, creature types, or abilities. It's colorless and has a mana value of 0. Other effects that apply to the creature can still grant it any of these characteristics.

Horde Ambusher — ruling 6

Any time you have priority, you may turn the face-down creature face up by revealing what its morph cost is and paying that cost. This is a special action. It doesn't use the stack and can't be responded to. Only a face-down permanent can be turned face up this way; a face-down spell cannot.

Horde Ambusher — ruling 7

Because the permanent is on the battlefield both before and after it's turned face up, turning a permanent face up doesn't cause any enters-the-battlefield abilities to trigger.

Horde Ambusher — ruling 8

A permanent that turns face up or face down changes characteristics but is otherwise the same permanent. Spells and abilities that were targeting that permanent, as well as Auras and Equipment that were attached to the permanent, aren't affected.

Horde Ambusher : règles, fonctionnement et interactions (MTG)

Horde Ambusher 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

Whenever this creature blocks, it deals 1 damage to you. Morph—Reveal a red card in your hand. (You may cast this card face down as a 2/2 creature for . Turn it face up any time for its morph cost.) When this creature is turned face up, target creature can't block this turn.

How it works : Horde Ambusher

Original English text — Wizards of the Coast / Scryfall

#1September 20, 2014· Wizards of the Coast
Turning Horde Ambusher face up after blockers have been declared won't remove any creature from combat or change how any creature is blocking.
No other card is named in this ruling.
#2September 20, 2014· Wizards of the Coast
If a face-down Horde Ambusher blocks and is then turned face up, its first ability won't trigger because it didn't have that ability when it blocked.
No other card is named in this ruling.
#3September 20, 2014· Wizards of the Coast
Morph lets you cast a card face down by paying , and lets you turn the face-down permanent face up any time you have priority by paying its morph cost.
Card named in this ruling
Morph
#4September 20, 2014· Wizards of the Coast
The face-down spell has no mana cost and has a mana value of 0. When you cast a face-down spell, put it on the stack face down so no other player knows what it is, and pay . This is an alternative cost.
No other card is named in this ruling.
#5September 20, 2014· Wizards of the Coast
When the spell resolves, it enters the battlefield as a 2/2 creature with no name, mana cost, creature types, or abilities. It's colorless and has a mana value of 0. Other effects that apply to the creature can still grant it any of these characteristics.
No other card is named in this ruling.
#6September 20, 2014· Wizards of the Coast
Any time you have priority, you may turn the face-down creature face up by revealing what its morph cost is and paying that cost. This is a special action. It doesn't use the stack and can't be responded to. Only a face-down permanent can be turned face up this way; a face-down spell cannot.
Card named in this ruling
Morph
#7September 20, 2014· Wizards of the Coast
Because the permanent is on the battlefield both before and after it's turned face up, turning a permanent face up doesn't cause any enters-the-battlefield abilities to trigger.
No other card is named in this ruling.
#8September 20, 2014· Wizards of the Coast
A permanent that turns face up or face down changes characteristics but is otherwise the same permanent. Spells and abilities that were targeting that permanent, as well as Auras and Equipment that were attached to the permanent, aren't affected.
No other card is named in this ruling.
#9September 20, 2014· Wizards of the Coast
At any time, you can look at a face-down spell or permanent you control. You can't look at face-down spells or permanents you don't control unless an effect instructs you to do so.
No other card is named in this ruling.
#10September 20, 2014· Wizards of the Coast
You must ensure that your face-down spells and permanents can easily be differentiated from each other. You're not allowed to mix up the cards that represent them on the battlefield in order to confuse other players. The order they entered the battlefield should remain clear. Common methods for doing this include using markers or dice, or simply placing them in order on the battlefield.
No other card is named in this ruling.
#11September 20, 2014· Wizards of the Coast
If a face-down permanent leaves the battlefield, you must reveal it. You must also reveal all face-down spells and permanents you control if you leave the game or if the game ends.
No other card is named in this ruling.