Sozin's Comet

When you cast Sozin's Comet and during your upkeep,
put a comet counter on it. If Sozin's comet already has five
or more comet counters on it during your upkeep, start
removing counters from it instead. Once you've removed the last counter, sacrifice Sozin's Comet.

Firebenders and Avatars you control have +1/+0 for every comet counter on Sozin' s Comet

"Only once in every one hundred years can a firebender experience this kind of power!"