Player 1 plays a spell that reads "Deal 4 to a unit at a battlefield. Discard 1. Draw 1. Move a friendly Unit from its base to a battlefield." Player 2 plays a spell that, before the resolution of that spell, moves the unit that was meant to be dealt 4 damage to its base. Given that Player 1 has no cards in hand, what will happen when Player 1's spell resolves?

When Player 1's spell resolves, it will continue to resolve as much as possible despite the initial target becoming illegal. Here is the breakdown of what happens:

Sequence of Events

1. Spell Finalization: Player 1 plays the spell targeting an enemy unit at a battlefield. The spell goes on the chain.
2. Reaction: Player 2 plays a spell moving the targeted unit from the battlefield to their base.
3. Chain Resolution (LIFO):
   • Player 2's spell resolves, moving the unit to their base.
   • Player 1's spell begins to resolve.

Resolution of Player 1's Spell

According to Core Rules Rule 359.3.e.1 and Rule 359.3.e.5, a spell resolves even if some of its targets are illegal, and instructions related to illegal targets are ignored.

1. "Deal 4 to a unit at a battlefield": The target unit is no longer at a battlefield. It is an illegal target, so this instruction is ignored. No damage is dealt.
2. "Discard 1": Player 1 has no cards in hand. Per Core Rules Rule 422.4, if instructed to discard more cards than they have, the instruction is ignored.
3. "Draw 1": As per Core Rules Rule 359.3.e.11 (and the precedent in RiftJudge FAQ #3134), if one instruction in a spell is ignored or cannot be followed, the spell continues to resolve the remaining instructions. Player 1 draws 1 card.
4. "Move a friendly Unit from its base to a battlefield": Player 1 may now choose a friendly unit at their base and move it to a battlefield, provided there is a legal destination (e.g., the battlefield is not at its unit capacity limit).

Summary: The spell will deal no damage, Player 1 will discard nothing, Player 1 will draw 1 card, and Player 1 will then perform the move if they have a legal unit and destination.