Dragon Whisperer
Creature — Human Shaman
: This creature gains [[ keyword: "flying" ]] until end of turn.
: This creature gets +1/+0 until end of turn.
Formidable — 
: Create a 4/4 red Dragon creature token with [[ keyword: "flying" ]]. Activate only if creatures you control have total power 8 or greater.
Chris Rallis
Dragons of Tarkir (DTK)
• #137Rulings
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.
(wotc • 2015-02-25)
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.
(wotc • 2015-02-25)
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.
(wotc • 2015-02-25)
Rulings
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.
(wotc • 2015-02-25)
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.
(wotc • 2015-02-25)
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.
(wotc • 2015-02-25)