Kassadin, the Void Walker

Allegiance - The Void (Whenever a creature with Allegiance - The Void enters the battlefield under your control, tap target creature.)
{u}{u}, Exile Kassadin, the Void Walker : Counter target spell.
If Kassadin, the Void Walker is in exile, you may
pay {w}{b}. If you do, return it to the battlefield
under your control.