Hi!
If Library of Leng is on battlefield and Hidden Horror EtB-ability resolve, can we put discarded card on top of library?
“Discard a card” is a cost in such kind of ability?
And Disciple of Deceit interaction with Library of Leng is also not so clear to me.
Thank you.

Nope, you can't do that. Since the card is never revealed, then the game cannot confirm that you have discarded a creature card, and the game forces you to rewind. If you want to discard a creature card for the Horror's trigger, you cannot choose to apply the Leng's effect to it, otherwise the game cannot see that you've discarded a creature card and the payment is illegal (as described in 701.7c).

Nathan Long
Wizards.Com Boards NetRep