Calamity Bearer — ruling 1

The damage is dealt by the same source as the original source of damage. The doubled damage isn’t dealt by Calamity Bearer unless it was the original source of damage.

Calamity Bearer — ruling 2

If another effect (or effects) modifies how much damage your Giant source would deal—by preventing some of it, for example—the player being dealt damage or the controller of the permanent being dealt damage chooses the order in which any such effects (including Calamity Bearer’s) apply. If all of the damage is prevented, Calamity Bearer’s effect no longer applies.

Calamity Bearer — ruling 3

If damage dealt by a Giant source you control is being divided or assigned among multiple permanents and/or players, that damage is divided or assigned before doubling. For example, if you attack with a 5/5 Giant with trample and it’s blocked by a 2/2 creature, you can assign 2 damage to the blocker and 3 damage to the defending player. Those amounts are then doubled to 4 and 6, respectively.

Calamity Bearer : règles, fonctionnement et interactions (MTG)

Calamity Bearer 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

If a Giant source you control would deal damage to a permanent or player, it deals double that damage to that permanent or player instead.

How it works : Calamity Bearer

Original English text — Wizards of the Coast / Scryfall

#1February 5, 2021· Wizards of the Coast
The damage is dealt by the same source as the original source of damage. The doubled damage isn’t dealt by Calamity Bearer unless it was the original source of damage.
No other card is named in this ruling.
#2February 5, 2021· Wizards of the Coast
If another effect (or effects) modifies how much damage your Giant source would deal—by preventing some of it, for example—the player being dealt damage or the controller of the permanent being dealt damage chooses the order in which any such effects (including Calamity Bearer’s) apply. If all of the damage is prevented, Calamity Bearer’s effect no longer applies.
No other card is named in this ruling.
#3February 5, 2021· Wizards of the Coast
If damage dealt by a Giant source you control is being divided or assigned among multiple permanents and/or players, that damage is divided or assigned before doubling. For example, if you attack with a 5/5 Giant with trample and it’s blocked by a 2/2 creature, you can assign 2 damage to the blocker and 3 damage to the defending player. Those amounts are then doubled to 4 and 6, respectively.
No other card is named in this ruling.