Does Doomsday resolve if the player doesn't have enough cards in their library to form a pile?

Doomsday instructs its controller to search their library, hand, and graveyard for five cards and put them on the bottom of their library in any order, then shuffle. If a player cannot assemble a full pile of five cards because they don't have enough cards across all three zones, the spell still resolves — they just use however many cards are available (even zero). Magic's rules never fail a spell simply because an instruction cannot be fully carried out.

This is grounded in CR 608.2d, which states that if an effect cannot be completed, the spell or ability does as much as it can. Doomsday's effect is applied to the extent possible, so the player forms a pile with whatever cards exist across library, hand, and graveyard, then the rest of the effect (losing half their life, rounded up) still applies regardless.

Crucially, the life-loss portion of Doomsday is not conditional on forming a full five-card pile. Both parts of the card's effect resolve: assemble the pile (with whatever is available) AND lose half your life. The life loss cannot be skipped simply because the pile was smaller than five cards.

Concrete example: You cast Doomsday when you have 2 cards in library, 1 in hand, and 1 in graveyard. Doomsday resolves: you search those zones, pick up to 5 cards (you find 4), arrange them at the bottom of your now-empty library, shuffle (no other cards to shuffle), then lose half your life rounded up. The spell fully resolved despite the incomplete pile.

Note that if the player has zero cards in all three zones, the pile is simply empty and the library remains empty — but the spell still resolves and the life loss still occurs (CR 608.2d, CR 608.2b).