Sundering Stroke — ruling 1

You must divide the damage as you announce Sundering Stroke, even if you intend to pay seven red mana to cast it. If you copy Sundering Stroke, no mana was spent to cast the copy at all, so the copy will deal the damage as divided.

Sundering Stroke — ruling 2

Each chosen target must receive at least 1 damage.

Sundering Stroke — ruling 3

If some of the targets become illegal for Sundering Stroke, the original division of damage still applies, but the damage that would have been dealt to illegal targets isn't dealt at all.

Sundering Stroke — ruling 4

If an effect allows you to cast Sundering Stroke without paying its mana cost, you can't choose to cast it and pay its cost unless another rule or effect allows you to cast it for a cost.

Sundering Stroke — ruling 5

Sundering Stroke checks what mana was actually spent to cast it. If an effect allows you to spend mana “as though it were mana” of any color or type, that will allow you to spend mana you couldn't otherwise spend, but it won't change what mana you spent to cast the spell.

Sundering Stroke : règles, fonctionnement et interactions (MTG)

Sundering Stroke 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

Sundering Stroke deals 7 damage divided as you choose among one, two, or three targets. If at least seven red mana was spent to cast this spell, instead Sundering Stroke deals 7 damage to each of those permanents and/or players.

How it works : Sundering Stroke

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2019· Wizards of the Coast
You must divide the damage as you announce Sundering Stroke, even if you intend to pay seven red mana to cast it. If you copy Sundering Stroke, no mana was spent to cast the copy at all, so the copy will deal the damage as divided.
No other card is named in this ruling.
#2October 4, 2019· Wizards of the Coast
Each chosen target must receive at least 1 damage.
No other card is named in this ruling.
#3October 4, 2019· Wizards of the Coast
If some of the targets become illegal for Sundering Stroke, the original division of damage still applies, but the damage that would have been dealt to illegal targets isn't dealt at all.
No other card is named in this ruling.
#4October 4, 2019· Wizards of the Coast
If an effect allows you to cast Sundering Stroke without paying its mana cost, you can't choose to cast it and pay its cost unless another rule or effect allows you to cast it for a cost.
No other card is named in this ruling.
#5October 4, 2019· Wizards of the Coast
Sundering Stroke checks what mana was actually spent to cast it. If an effect allows you to spend mana “as though it were mana” of any color or type, that will allow you to spend mana you couldn't otherwise spend, but it won't change what mana you spent to cast the spell.
No other card is named in this ruling.