Nico Bolas's Crown.

Amass 10
Proliferate, then proliferate again, then again, and again, and again.
If you have a Nico Bolas planeswalker, in addition, poliferate, then poliferate again, then again, and again, and again and attach Nico Bolas's crown to that Nico Bolas planeswalker.
Planeswallkers attached can attack.