Dark Magician the Chaos Seeker

Dark Magician the Chaos Seeker enters the battlefield with X +1/+1 counters.
Whenever you cast an instant or sorcery, you may put a +1/+1 counter on Dark Magician the Chaos Seeker.
At the beginning of your end step if there are ten or more +1/+1 counters on Dark Magician the Chaos Seeker, transform it.