Rem Karolus, Stalwart Slayer — ruling 1

Rem Karolus, Stalwart Slayer's last two abilities apply only to damage dealt by spells on the stack. A spell that becomes a permanent and then deals damage won't be affected.

Rem Karolus, Stalwart Slayer — ruling 2

If another effect modifies how much damage a spell would deal to an opponent or a permanent they control, including preventing some of it, the player being dealt damage or the controller of the permanent being dealt damage chooses an order in which to apply those effects. If all of the damage is prevented, Rem's effect no longer applies.

Rem Karolus, Stalwart Slayer — ruling 3

The additional 1 damage is dealt by the original source of damage. The additional damage isn't dealt by Rem Karolus.

Rem Karolus, Stalwart Slayer — ruling 4

If damage dealt by a source you control is being divided or assigned among multiple permanents an opponent controls or among an opponent and one or more permanents they control, divide the original amount before adding 1. For example, if you cast a spell that deals 4 damage divided among any number of targets and you choose to have it deal 2 damage to a creature and 2 damage to that creature's controller, that spell will deal 3 damage to each of them instead.

Rem Karolus, Stalwart Slayer — ruling 5

Some spells cause other objects to deal damage, such as spells that cause two creatures to fight. Rem Karolus, Stalwart Slayer's last two abilities don't apply to damage from such effects.

Rem Karolus, Stalwart Slayer : règles, fonctionnement et interactions (MTG)

Rem Karolus, Stalwart Slayer 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

Flying, haste If a spell would deal damage to you or another permanent you control, prevent that damage. If a spell would deal damage to an opponent or a permanent an opponent controls, it deals that much damage plus 1 instead.

How it works : Rem Karolus, Stalwart Slayer

Original English text — Wizards of the Coast / Scryfall

#1September 24, 2021· Wizards of the Coast
Rem Karolus, Stalwart Slayer's last two abilities apply only to damage dealt by spells on the stack. A spell that becomes a permanent and then deals damage won't be affected.
No other card is named in this ruling.
#2September 24, 2021· Wizards of the Coast
If another effect modifies how much damage a spell would deal to an opponent or a permanent they control, including preventing some of it, the player being dealt damage or the controller of the permanent being dealt damage chooses an order in which to apply those effects. If all of the damage is prevented, Rem's effect no longer applies.
No other card is named in this ruling.
#3September 24, 2021· Wizards of the Coast
The additional 1 damage is dealt by the original source of damage. The additional damage isn't dealt by Rem Karolus.
No other card is named in this ruling.
#4September 24, 2021· Wizards of the Coast
If damage dealt by a source you control is being divided or assigned among multiple permanents an opponent controls or among an opponent and one or more permanents they control, divide the original amount before adding 1. For example, if you cast a spell that deals 4 damage divided among any number of targets and you choose to have it deal 2 damage to a creature and 2 damage to that creature's controller, that spell will deal 3 damage to each of them instead.
No other card is named in this ruling.
#5September 24, 2021· Wizards of the Coast
Some spells cause other objects to deal damage, such as spells that cause two creatures to fight. Rem Karolus, Stalwart Slayer's last two abilities don't apply to damage from such effects.
No other card is named in this ruling.