Whammy Burn — ruling 1

If you don’t have a whammy deck when an effect refers to it, shuffle a Plains, Island, Swamp, Mountain, and Forest card from outside the game to form your whammy deck immediately before performing any part of that effect.

Whammy Burn — ruling 2

The whammy deck exists in the additional deck zone. Since this is a game zone, it’s not outside the game (for effects such as that of Frontier Explorer).

Whammy Burn — ruling 3

You can’t draw cards from the whammy deck or interact with it in any way other than by effects that specifically interact with the whammy deck. Effects that affect cards you own not on the battlefield don’t affect cards in the whammy deck.

Whammy Burn — ruling 4

You choose a target as you cast Whammy Burn, before revealing any cards from your whammy deck.

Whammy Burn : règles, fonctionnement et interactions (MTG)

Whammy Burn 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

Shuffle your whammy deck, then reveal cards from the top of your whammy deck until you reveal an Island or choose to stop. Then, if no Islands were revealed this way, Whammy Burn deals X damage to any target, where X is the number of cards revealed. (A whammy deck consists of a Plains, Island, Swamp, Mountain, and Forest.)

How it works : Whammy Burn

Original English text — Wizards of the Coast / Scryfall

#1November 12, 2019· Wizards of the Coast
If you don’t have a whammy deck when an effect refers to it, shuffle a Plains, Island, Swamp, Mountain, and Forest card from outside the game to form your whammy deck immediately before performing any part of that effect.
No other card is named in this ruling.
#2November 12, 2019· Wizards of the Coast
The whammy deck exists in the additional deck zone. Since this is a game zone, it’s not outside the game (for effects such as that of Frontier Explorer).
No other card is named in this ruling.
#3November 12, 2019· Wizards of the Coast
You can’t draw cards from the whammy deck or interact with it in any way other than by effects that specifically interact with the whammy deck. Effects that affect cards you own not on the battlefield don’t affect cards in the whammy deck.
No other card is named in this ruling.
#4November 12, 2019· Wizards of the Coast
You choose a target as you cast Whammy Burn, before revealing any cards from your whammy deck.
No other card is named in this ruling.