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From MTG Wiki
Formidable is an ability word introduced in Dragons of Tarkir, where it is the signature ability of the red-green Clan Atarka. Creatures with formidable all care in some way about controlling creatures with total power 8 or greater.  
- 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.
- 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.
- 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 -
Creature - Elk
Formidable — When Stampeding Elk Herd attacks, if creatures you control have total power 8 or greater, creatures you control gain trample until end of turn.