Descendants' Path
At the beginning of your upkeep, reveal the top card of your library. If it's a creature card that shares a creature type with a creature you control, you may cast it without paying its mana cost. If you don't cast it, put it on the bottom of your library.
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Official rulings (3) · Scryfall
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.
If the card has {X} in its mana cost, you must choose 0 as its value.