Keldon Flamesage — ruling 1

If Keldon Flamesage leaves the battlefield in response to its triggered ability, use its power as it last existed on the battlefield to determine the value of X.

Keldon Flamesage — ruling 2

If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs, such as a kicker cost. If the card has any mandatory additional costs, you must pay those to cast the spell.

Keldon Flamesage — ruling 3

If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Keldon Flamesage — ruling 4

The attacking player chooses whether to tap a creature for an enlist ability immediately after they tap the creatures that they have chosen to attack with. You can't choose to enlist a creature later.

Keldon Flamesage — ruling 5

To enlist a creature, that creature must be untapped, it must not be attacking (even if it has vigilance), and it must have haste or have been under that attacking player's control since the beginning of their current turn.

Keldon Flamesage — ruling 6

When a player taps a creature for an attacking creature's enlist ability, that attacking creature gets +X/+0 until end of turn, where X is the tapped creature's power. This is a triggered ability that goes on the stack immediately after attackers have been declared in the declare attackers step.

Keldon Flamesage — ruling 7

You may tap only one creature for an enlist ability of an attacking creature, and a single creature can't be tapped for more than one enlist ability.

Keldon Flamesage : règles, fonctionnement et interactions (MTG)

Keldon Flamesage 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

Enlist Whenever this creature attacks, look at the top X cards of your library, where X is this creature's power. You may exile an instant or sorcery card with mana value X or less from among them. Put the rest on the bottom of your library in a random order. You may cast the exiled card without paying its mana cost.

How it works : Keldon Flamesage

Original English text — Wizards of the Coast / Scryfall

#1September 9, 2022· Wizards of the Coast
If Keldon Flamesage leaves the battlefield in response to its triggered ability, use its power as it last existed on the battlefield to determine the value of X.
No other card is named in this ruling.
#2September 9, 2022· Wizards of the Coast
If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs, such as a kicker cost. If the card has any mandatory additional costs, you must pay those to cast the spell.
No other card is named in this ruling.
#3September 9, 2022· Wizards of the Coast
If the card has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
No other card is named in this ruling.
#4September 9, 2022· Wizards of the Coast
The attacking player chooses whether to tap a creature for an enlist ability immediately after they tap the creatures that they have chosen to attack with. You can't choose to enlist a creature later.
No other card is named in this ruling.
#5September 9, 2022· Wizards of the Coast
To enlist a creature, that creature must be untapped, it must not be attacking (even if it has vigilance), and it must have haste or have been under that attacking player's control since the beginning of their current turn.
No other card is named in this ruling.
#6September 9, 2022· Wizards of the Coast
When a player taps a creature for an attacking creature's enlist ability, that attacking creature gets +X/+0 until end of turn, where X is the tapped creature's power. This is a triggered ability that goes on the stack immediately after attackers have been declared in the declare attackers step.
No other card is named in this ruling.
#7September 9, 2022· Wizards of the Coast
You may tap only one creature for an enlist ability of an attacking creature, and a single creature can't be tapped for more than one enlist ability.
No other card is named in this ruling.