Activate Timestream Navigator by paying ({2} magic symbol) ({U} magic symbol) ({U} magic symbol) , tapping it and putting it on the bottom of your library, causing you to take an extra turn after this one.
Activate Cellar Door by paying ({3} magic symbol) and tapping it, causing you to put Timestream Navigator from the bottom of your library into your graveyard and create a 2/2 Zombie creature token.
Grolnok triggers, exiling Timestream Navigator with a croak counter on it.
Cast Timestream Navigator from exile by paying ({1} magic symbol) ({U} magic symbol) .