Ashiok, Wicked Nightmare's first ability isn't optional. You can't choose to pay life instead of exiling cards from the top of your library while you control Ashiok, and you can't split the payment between life and cards.
2023-09-01
If you would pay life while you control Ashiok and your library does not have at least that many cards in it, you'll just pay life as normal.
2023-09-01
Ashiok's first ability doesn't allow you to attempt to pay an amount of life greater than your current life total.
If you would pay life while your library has at least that many cards in it, exile that many cards from the top of your library instead.
+1: Look at the top two cards of your library. Exile one of them and put the other into your hand.
−2: Create two 1/1 black Nightmare creature tokens with "At the beginning of combat on your turn, if a card was put into exile this turn, put a +1/+1 counter on this token."
−7: Target player exiles the top X cards of their library, where X is the total mana value of cards you own in exile.
