Crater Elemental — ruling 1

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.

Crater Elemental — ruling 2

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.

Crater Elemental — ruling 3

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.

Crater Elemental — ruling 4

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.

Crater Elemental — ruling 5

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.

Crater Elemental : règles, fonctionnement et interactions (MTG)

Crater Elemental 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

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

How it works : Crater Elemental

Original English text — Wizards of the Coast / Scryfall

#1February 25, 2015· Wizards of the Coast
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.
No other card is named in this ruling.
#2February 25, 2015· Wizards of the Coast
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.
No other card is named in this ruling.
#3February 25, 2015· Wizards of the Coast
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.
No other card is named in this ruling.
#4February 25, 2015· Wizards of the Coast
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.
No other card is named in this ruling.
#5February 25, 2015· Wizards of the Coast
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.
No other card is named in this ruling.