Crater Elemental

{R}, {T}, Sacrifice this creature: It deals 4 damage to target creature.
Formidable — {2}{R}: This creature has base power 8 until end of turn. Activate only if creatures you control have total power 8 or greater.

Dragons of Tarkir

{R}, {T}, Sacrifice Crater Elemental: Crater Elemental deals 4 damage to target creature.
Formidable — {2}{R}: Crater Elemental has base power 8 until end of turn. Activate this ability only if creatures you control have total power 8 or greater.

Dragons of Tarkir Promos

Crater Elemental's last ability overrides all previous effects that set its power to a specific value. Other effects that set its power to a specific value that start to apply after the ability resolves will override this effect.

Effects that modify Crater Elemental's power, such as the effects of Giant Growth or Hall of Triumph, will apply to it no matter when they started to take effect. The same is true for counters that change its power (such as +1/+1 counters) and effects that switch its power and toughness.

If you control a creature with power less than 0, use its actual power when calculating the total power of creatures you control. For example, if you control three creatures with powers 4, 5, and -2, the total power of creatures you control is 7.

Other formidable abilities are triggered abilities with an "intervening 'if'" clause. Such abilities check the total power of creatures you control twice: once at the appropriate time to see if the ability will trigger, and again as the ability tries to resolve. If, at that time, the total power of creatures you control is no longer 8 or greater, the ability will have no effect.

Some formidable abilities are activated abilities that require creatures you control to have total power 8 or greater. Once you activate these abilities, it doesn't matter what happens to the total power of creatures you control.