Stampeding Elk Herd — ruling 1

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.

Stampeding Elk Herd — ruling 2

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.

Stampeding Elk Herd — ruling 3

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.

Stampeding Elk Herd : règles, fonctionnement et interactions (MTG)

Stampeding Elk Herd 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

Formidable — Whenever this creature attacks, if creatures you control have total power 8 or greater, creatures you control gain trample until end of turn.

How it works : Stampeding Elk Herd

Original English text — Wizards of the Coast / Scryfall

#1February 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.
#2February 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.
#3February 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.