Ob Nixilis, the Robbed

Whenever an opponent draws a card, put a +1/+1 counter on Ob Nixilis, the Robbed.
{2}{b}: Target player draws a card, then Ob Nixilis gets +X/+X until end of turn for each card
an opponent has drawn this turn.
His Spark stolen, his pride humbled, his might
crushed, Ob Nixilis rushed to Kaladesh to reclaim
what he had lost.