356.3.e. Handling illegal and impossible instructions
356.3.e.1. The spell resolves even if some or all of its targets are illegal.
356.3.e.2. A target is illegal as the spell resolves if it no longer meets the targeting requirements of the spell, or if it has changed Zones to or from a Non-Board Zone.
356.3.e.3. If a target ceases to meet the targeting requirements while the spell is on the chain, then meets them again, it's a legal target. Example: A spell targets "a unit at a battlefield." A player reacts with a spell that moves the unit to base, then another player reacts with a spell that moves it back to that battlefield, then the original spell resolves. The unit is a legal target.
356.3.e.4. If a target changes Zones to or from a Non-Board Zone and then returns to its original zone, it is no longer a legal target, because it's not treated as the same object. Examples: An enemy unit at a battlefield is no longer a legal target if it is no longer an enemy, no longer a unit, or no longer at a battlefield. A unit with 3 or less Might is no longer a legal target if it is no longer a unit or if its Might is greater than 3. Something that's exhausted is no longer a legal target if it is no longer exhausted. (It can't stop being "something.") A spell that's played from hidden can normally only target its own battlefield or something at that battlefield. A target for such a spell may cease to be a legal target if it moves from the battlefield where that spell was played, even if the spell has no location targeting requirement otherwise.
356.3.e.5. If any of the spell's targets are no longer legal, those targets are unaffected by the spell as it resolves. Example: A player plays Void Seeker, a spell that says "Deal 4 to a unit at a battlefield. Draw 1." The unit's controller uses a Reaction to move the unit to their base. Since the unit is no longer a legal target, it is not dealt any damage. Void Seeker's controller still draws 1.
356.3.e.6. Instructions that can't be followed, either because of illegal targets or other circumstances, are ignored.
356.3.e.7. If all of an instruction's Targets become Invalid or Unavailable by the time the spell is finished being played, that instruction will not execute.
356.3.e.8. If an instruction has more than one Target and fewer than all of the Targets become Invalid or Unavailable by the time the spell is finished being played, the instruction will execute, with only the Targets available and valid being operated on.
356.3.e.9. The process for a card's choice becoming Invalid or Unavailable is referred to as mistargeting. Example: A spell has the instruction "Deal 2 to a unit at a battlefield." Before that instruction can execute, the chosen unit is moved to its base. The instruction will not be executed, because it specifies that the unit it chooses must be at a Battlefield, and by the time it attempted to execute, the unit was no longer valid as a choice.
356.3.e.10. It is possible for none of a spell's instructions to be executed as it resolves, due to all of them requiring targets to act on and all of those targets becoming Invalid or Unavailable . In this case, the spell has no effect but is still considered played. Example: A player plays a spell that reads "Deal 2 to a unit at a battlefield" with no other instructions, and chooses an enemy unit at a battlefield. They also control a unit with the ability "When you play a spell, give me +1 [M] this turn." Before the spell resolves, the chosen unit is moved to its base. The spell resolves and its only instruction cannot be executed, but the unit's ability still triggers as the spell resolves and gives it +1 [M].
356.3.e.11. Instructions that can be partially followed are followed as much as possible and ignored otherwise. Example: A player plays a spell that says "Discard 2, then draw 2" If their hand is empty, the instruction to discard 2 will be ignored. They'll still draw 2. If they had 1 card in hand, they would discard it and draw 2.
356.3.e.12. If the spell checks information about a target that is no longer legal or a card or permanent whose location, zone, or status has changed such that that information is no longer available, that check returns "null" and all calculations based on it are ignored. Examples: A unit that is no longer on the board is treated as having null Might, null cost, etc. A unit that is no longer on the board has no location, is neither exhausted nor readied, etc. Baited Hook says "[1][C], [E]: Kill a friendly unit. Look at the top 5 cards of your Main Deck. You may banish a unit from among them that has Might up to 1 more than the killed unit and play it, ignoring its cost. Then recycle the rest." While Baited Hook’s ability is on the chain, an opponent reacts with a spell that returns the friendly unit to its owner's hand. Because the friendly unit is no longer a legal target, it can't be killed and its Might is treated as null. Baited Hook’s controller reveals the top 5 cards of their Main Deck, but can’t choose any unit from among them.
356.3.e.13. A spell or ability that moves something to a different zone as a cost or effect can "look back" at its characteristics before it changes zones.