Thuriok, Knight of Ill Omen

First strike, menace, haste
Whenever a creature your opponent controls
dies, put a -1/-1 counter on Thuriok.
When Thuriok dies, spells you cast this turn cost
{x} less to cast, where X is the number of -1/-1
counters on Thuriok.
"Farewell. We will likely meet again, should destiny dictate."—Unknown