Karthus, The Death Singer

Whenever a creature an opponent controls dies,
add {u} or {b} to your mana pool. This mana does
not empty your pool as phases end. Spend this
mana only to activate abilities of permanents you
control.
{u}{u}{u}{u}{b}{b}{b}{b}{t}: Karthus deals 40 damage
to each opponent and each creature they control.
spend only mana created by creatures to activate
this ability. activate this ability only anytime you
could cast a sorcery.
"Death is a song all will hear."