Exile all cards from target player's library then player shuffles hand into library

[Samus]

I dont understand this ability from worldwake jace:

Exile all cards from target player's library, then that player shuffles his or her hand into his or her library.

if you exiled all the library then the player no longer has a library so how can he shuffle his cards into it

[Arix]

You always have a library, regardless of whether or not there are any cards in it. Even if there are zero cards in your library, you still have a library zone.

Basically the ability just means that you exile all the cards from someone's library, they shuffle their hand, and that hand becomes their new library.

[Samus]

What if theres also no cards in the hand?

[Arix]

Same deal. You always have a hand, even if there are no cards in it.

If they have no cards in their library or hand when you activate that ability...well...sucks to be them.

[Murloc]

What would happen if that player had a Lich's Mirror in play?

[Arix]

The next time they had to draw, they'd lose the game. This causes Lich's Mirror to kick in, shuffle up their permanents and graveyard to form a new library, then draw seven cards.