Fettergeist
Creature — Spirit
Flying
At the beginning of your upkeep, sacrifice this creature unless you pay 
for each other creature you control.
The choices of the living will tether them in death.
Izzy
Avacyn Restored (AVR)
• #52Rulings
The amount of mana you need to pay is determined when the triggered ability resolves.
(wotc • 2012-05-01)
Rulings
The amount of mana you need to pay is determined when the triggered ability resolves.
(wotc • 2012-05-01)