Corvus, the Black Raven

When Corvus, the Black Raven enters the battlefield, Chose a player.
Every time the Chosen player casts a spell,
put X Dark counters on all creatures they control.
Every time chosen player exiles a creature,
put a +1/+1 counter on Corvus, the Black Raven.
If Corvus, the Black Raven has 5 or more +1/+1 counter on it, it has hexproof