Maganam's Infectious Blast

(You may cast a legendary instant only if you control
a legendary creature or planeswalker.)
Wither
Cast this spell only when a creature dies. Exile
that creature instead. This spell deals damage
equal to that creature's toughness to all creatures
and planeswalkers.
You may repeat this process each time a creature
dies to this spell. Whenever a creature or
planeswalker dies to this spell, you gain 3 life.