Slaying Fire — ruling 1

If an instant or sorcery spell has an adamant ability, you perform the spell's instructions in order. You don't perform the adamant instruction before the spell resolves or before any other effects printed above it.

Slaying Fire — ruling 2

If an effect allows you to cast a spell without paying its mana cost, you can't choose to cast it and pay unless another rule or effect allows you to cast that spell for a cost. Similarly, you can't waive a cost reduction unless that effect says you may.

Slaying Fire — ruling 3

Adamant effects check what mana was actually spent to cast a spell. If an effect allows you to spend mana “as though it were mana” of any color or type, that allows you to spend mana you couldn't otherwise spend, but it doesn't change what mana you spent to cast the spell.

Slaying Fire — ruling 4

If you copy a spell that has an adamant ability, no mana was spent to cast the copy at all, so that ability won't apply.

Slaying Fire : règles, fonctionnement et interactions (MTG)

Slaying Fire 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

Slaying Fire deals 3 damage to any target. Adamant — If at least three red mana was spent to cast this spell, it deals 4 damage instead.

How it works : Slaying Fire

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2019· Wizards of the Coast
If an instant or sorcery spell has an adamant ability, you perform the spell's instructions in order. You don't perform the adamant instruction before the spell resolves or before any other effects printed above it.
No other card is named in this ruling.
#2October 4, 2019· Wizards of the Coast
If an effect allows you to cast a spell without paying its mana cost, you can't choose to cast it and pay unless another rule or effect allows you to cast that spell for a cost. Similarly, you can't waive a cost reduction unless that effect says you may.
No other card is named in this ruling.
#3October 4, 2019· Wizards of the Coast
Adamant effects check what mana was actually spent to cast a spell. If an effect allows you to spend mana “as though it were mana” of any color or type, that allows you to spend mana you couldn't otherwise spend, but it doesn't change what mana you spent to cast the spell.
No other card is named in this ruling.
#4October 4, 2019· Wizards of the Coast
If you copy a spell that has an adamant ability, no mana was spent to cast the copy at all, so that ability won't apply.
No other card is named in this ruling.