Roil's Retribution — ruling 1

You choose how many targets Roil’s Retribution has and how the damage is divided as you cast the spell. Each target must receive at least 1 damage.

Roil's Retribution — ruling 2

If some, but not all, of the targets become illegal by the time Roil’s Retribution tries to resolve, the original division of damage still applies, but no damage is dealt to illegal targets. If all the targets are illegal, the spell won’t resolve.

Roil's Retribution : règles, fonctionnement et interactions (MTG)

Roil's Retribution 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

Roil's Retribution deals 5 damage divided as you choose among any number of target attacking or blocking creatures.

How it works : Roil's Retribution

Original English text — Wizards of the Coast / Scryfall

#1August 25, 2015· Wizards of the Coast
You choose how many targets Roil’s Retribution has and how the damage is divided as you cast the spell. Each target must receive at least 1 damage.
No other card is named in this ruling.
#2August 25, 2015· Wizards of the Coast
If some, but not all, of the targets become illegal by the time Roil’s Retribution tries to resolve, the original division of damage still applies, but no damage is dealt to illegal targets. If all the targets are illegal, the spell won’t resolve.
No other card is named in this ruling.