Group targets, legal choices and referents

Most targeting asks for one object at a time, but some cards ask for a group that has to meet a requirement as a whole, such as "any number of units at a battlefield with total Might 4 or less." When you finalize such a card you may choose any group of valid targets that together fulfill the requirement (rule 355.11 and rule 355.11.a). "Any number" and "up to" include zero, and a card played with zero targets is allowed (rule 355.13).

The trap comes on resolution. Reactions can change the group, and the requirement is checked again as the card resolves. If the group no longer meets it, the controller must choose a legal subset of the original targets that does, and they cannot add units they did not choose in the first place (rule 355.11.b). The subset has to be one you could not add to: you cannot leave out an original target that would still fit under the limit (rule 355.11.b.1). Different subsets of different sizes can both be legal, so with Might 4, 1, 1, 1 and a limit of 4 you may kill the three 1 Might units or the 4 Might unit alone. Here is a worked example. Fox-Fire kills any number of units at a battlefield with total Might 4 or less. You choose three units with Might 2, 1 and 1, which total 4. In reaction your opponent gives the 2 Might unit +1, so the Mights are 3, 1 and 1 and the total is 5. The group fails, so you must pick a legal subset that you could not add another chosen unit to. Every pair adds up to 4 or less, so any of the three pairs works. Killing only one unit is not allowed, because another chosen unit would still fit, and neither is adding a unit you had not chosen.

Two more rules protect the choices you make while playing a card. A destination for a Move must be a location other than the unit's current one where it is allowed to be present (rule 355.4.a). And you may not make a choice that will deterministically lead to an illegal choice or action later in the process unless you have no alternative (rule 355.16 and rule 357.3). If a spell makes you kill a friendly unit as a cost and also gives a friendly unit +2 Might, the unit you pick for the bonus cannot be the one you plan to kill, because the bonus would then have nothing to land on.

Referents are the other half of this lesson. A word like "here", "my" or "its" takes its meaning from the source of the ability or from one of its targets (rule 359.3.f.1). By default that information is checked when the instruction executes, not when the ability went on the chain (rule 359.3.f.2). So if a reaction changes a unit's Might before its ability resolves, the ability uses the new Might. If the information is no longer available, for example because the unit has left the board, the check returns "null" and the instructions based on it are ignored (rule 359.3.e.12 and rule 359.3.f.2.a). There is an exception. Information that a triggered ability takes from its own trigger condition is checked when the trigger condition is fulfilled, and it stays fixed after that (rule 359.3.f.3). Lillia, Fae Fawn plays a Sprite "there" after moving from a location, and "there" means the location she left when the ability triggered, whatever happens to her next. Words like "enemy" and "friendly" in an ability's effect are measured from the triggered ability itself and are checked on execution (rule 359.3.f.4).