Keldon Flamesage

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.

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.

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.

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.

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.

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.