Grand Finale is a rare Silent Attack that costs 0 energy: "Can only be played if there are no cards in your Draw Pile. Deal 60 damage to ALL enemies." Upgraded, it deals 75.

When the draw pile reaches zero

Whenever you need to draw and your draw pile is empty, the game shuffles your discard pile into it first. So the draw pile is at zero only right after you draw its last card, before anything else makes you draw again.

With 5 cards drawn per turn and no other draw, the draw pile is empty after your turn's draw only when your deck size is a multiple of 5:

Deck sizeDraw pile is empty after the draw on
10Turn 2, 4, 6...
15Turn 3, 6, 9...
20Turn 4, 8, 12...
12Never without extra draw

Grand Finale also has to be in your hand at that moment. A card you drew earlier and did not play is discarded at the end of your turn, so it only comes back after the next shuffle.

Silent cards that change your draw count

CardCostEffect
Prepared0Draw 1 (2), discard 1 (2)
Acrobatics1Draw 3 (4), discard 1
Expertise1Draw until you have 6 (7) cards in hand
Calculated Gamble0Discard your hand, then draw that many cards. Exhaust
Reflex3Sly. Draw 2 (3)

Upgraded values are in brackets. Each extra card drawn moves the empty point one card earlier. If a draw card would draw more cards than your draw pile holds, the discard pile is shuffled in and the draw pile is no longer empty.

Your hand holds at most 10 cards; draws beyond that stop.

See the Silent page for the full card list.