Xantcha, Urza's Ravager

({p/b} can be paid with either {b} or 2 life.)
Xantcha, Urza's Ravager attacks each turn if able unless its controller pays 2 life.
Whenever a creature dies, you may pay 2 life. If you do, put a +1/+1 counter on Xantcha, Urza's Ravager.
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"Waste not, want not."