Dragon Throne of Tarkir
Legendary Artifact — Equipment
Equipped creature has defender and "
, 
: Other creatures you control gain trample and get +X/+X until end of turn, where X is this creature's power."
Equip 
What once soared high above Tarkir is now reduced to a seat.
Daarken
Rulings
If the equipped creature's power is negative while resolving the activated ability it gains, X is considered to be 0.
(wotc • 2017-07-14)
The value of X is determined as the activated ability resolves. If the equipped creature isn't on the battlefield at that time, use its power the last time it was on the battlefield to determine the value of X.
(wotc • 2014-09-20)
Rulings
If the equipped creature's power is negative while resolving the activated ability it gains, X is considered to be 0.
(wotc • 2017-07-14)
The value of X is determined as the activated ability resolves. If the equipped creature isn't on the battlefield at that time, use its power the last time it was on the battlefield to determine the value of X.
(wotc • 2014-09-20)