Theorist's Sanctum

Some spells and abilities that empower Jace may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. You won't empower Jace.

If you control a Jace planeswalker token, you can't choose to create a new one when you empower Jace. You have to put the loyalty counters on your existing Jace planeswalker token. In the rare case that you control multiple Jace planeswalker tokens, you'll choose one of those tokens to put the loyalty counters on.

Empower Jace won't let you put loyalty counters on nontoken Jace planeswalkers. However, if you control a Jace planeswalker token that you got another way (such as an effect that creates a token that's a copy of a permanent), empower Jace will put loyalty counters on that.