If you are unable to cast the card, perhaps because there are no legal targets available, the card remains in its owner's graveyard.

You may pay additional costs, such as kicker costs, of the instant or sorcery card.

If the card has any mandatory additional costs, then you must pay them in order to cast the spell.

If you reveal more than one Chancellor of the Spires from your opening hand, each opponent will put the top seven cards of their library into their graveyard that many times (for a total of fourteen, twenty-one, or twenty-eight cards).

The instant or sorcery card you target with the "enters" triggered ability is cast as part of the resolution of that ability. Timing restrictions based on the card's type are ignored. Other restrictions, such as "Cast [this card] only during combat," are not.

A player's "opening hand" is the hand of cards the player has after all players have taken mulligans. If players have any cards in hand that allow actions to be taken with them from a player's opening hand, the starting player takes all such actions first in any order, followed by each other player in turn order. Then the first turn begins.