FOUR!

(You may cast a legendary instant only if you control
a legendary creature or planeswalker.)
FOUR! deals X damage to target player or planeswalker times 1.65 rounded down, X can be
0. Then, after damage calculations, FOUR! does
an additional 1 damage for every 4 missing health
(Missing health is equal to 20 minus current health for players, or starting loyalty counters minus current
loyalty counters. Missing health can't be negative.)