Descendants' Path

If the card has {X} in its mana cost, you must choose 0 as its value.

If the revealed card is a creature card that shares a creature type with a creature you control, but you choose not to cast it, it's put on the bottom of your library. (This is a recent change.)

If you cast a card "without paying its mana cost," you can't pay any alternative costs. You can pay additional costs, such as kicker costs. If the card has mandatory additional costs, you must pay those.