I'm confused about some aspects of the Antilife Shell spell, which reads as follows:
Antilife Shell
School abjuration; Level cleric/oracle 6, druid 6, shaman 6; Domain
animal 6; Subdomain souls 6CASTING
Components V, S, DF Casting Time 1 round
EFFECT
Range 10 ft. Area 10-ft.-radius emanation, centered on you
Duration 1 min./level (D)
Saving Throw none; Spell Resistance yesDESCRIPTION
You bring into being a mobile, hemispherical energy field that
prevents the entrance of most types of living creatures.The effect hedges out animals, aberrations, dragons, fey, giants,
humanoids, magical beasts, monstrous humanoids, oozes, plants, and
vermin, but not constructs, elementals, outsiders, or undead.This spell may be used only defensively, not aggressively. Forcing an
abjuration barrier against creatures that the spell keeps at bay
collapses the barrier.
Since the spell has a casting time of 1 round, there's a risk that the enemies you're trying to hedge out will close to melee range before you've finished casting. I'm trying to figure out what the line "Forcing an abjuration barrier against creatures that the spell keeps at bay collapses the barrier." means in this case. Does the barrier immediately fail, or does the caster have the option not to force the barrier into the occupied squares? If the caster backs up, does the barrier prevent enemies from the closing the distance a second time?
This question is inspired by the Kingmaker adventure path, which features an lone caster enemy whose published tactics say to use this spell in a 20 ft. diameter room. RAW, I don't see how the spell can possibly be effective – even if the enemy takes the time to turn invisible first, the odds of there being a space in the room where the spell can be cast without immediately collapsing are essentially nil.
Best Answer
For convenience I've slightly modified the issues the question raises. I hope that's okay.
What does Forcing an abjuration barrier against creatures that the spell keeps at bay collapses the barrier means for a spell like the antilife shell spell? It means that, once the spell's effect is in place and the caster moves, the effect won't push around the battlefield appropriate creatures. Instead, when the caster moves and the effect would seem to mandate pushing an appropriate creature around the battlefield, the effect ends. On Abjuration says
The typical caster can't opt to shrink or reshape the antilife shell effect, so the caster must consider his movement carefully lest he inadvertently end the spell by trying to force a creature against the barrier the spell antilife shell creates.
By the way, the spell doesn't care about creatures in the area when the effect comes into being. The effect doesn't, for example, push away from the caster appropriate creatures that are already present nearby. Instead, the effect comes into being past such creatures, putting them inside the effect's area. The spell only prevents new appropriate creatures from entering the area; when the effect comes into being, it has no effect on creatures already in its area.
A caster casts the antilife shell while the caster is adjacent to a foe. Can the caster simply back up so that the foe is outside the effect, preventing the foe from closing with the caster on the foe's turn? O, yes! Absolutely this is a thing.
What if the antilife shell spell is cast in a 20-ft.×20-ft. room? This is complicated by the game not clearly explaining from where emanations emanate. An emanation is usually centered on the caster or a target, yet areas are always centered on a grid intersection. The typical reading seems have the emanation emanate from each of the intersections of the caster's space. (Also see Aiming a Spell on Burst, Emanation, or Spread and, for Pathfinder's antecedent D&D 3.5 this question.)
With this in mind, were Ⓐ to cast the spell antilife shell in a 20-ft.×20-ft. room, the effect could come into being like this:
(The antilife shell effect's circumference represented by magenta lines; the grid intersections from which the emanation emanates are the magenta dots.) This would see Ⓑ inside the effect but Ⓒ, ①, and ② unable to pass beyond the spell's effect. The effect would end were Ⓐ to take a 5-ft. step south as the effect tries and fails to hedge out Ⓒ. Similarly, the spell's effect could come into being like this:
This would see Ⓑ and Ⓒ inside the effect and ① and 2 again unable to pass beyond the spell's effect. Here also were Ⓐ to take a 5 ft. step south, the effect would end as the effect tries and fails to hedge out ②. Finally, the spell's effect could come into being like this:
This would again see Ⓑ and Ⓒ inside the effect and ① and ② still unable to pass beyond the spell's effect, but a 5-ft. step by Ⓐ in any normal direction won't end the spell effect.
Finally, because the spell antilife shell is an abjuration spell and not a spell of the conjuration school that brings forth an object or creature, barriers occlude the antilife shell effect but they don't end the effect. On Conjuration says
But these rules do not apply to effects of the abjuration school like the effect created by the spell antilife shell (or, for that matter, a prismatic sphere spell, for example). A caster needn't worry about wasting an abjuration spell—the spell's effect failing to come into being—because there's insufficient room for the spell's effect to reach maximum efficacy. Likewise, the caster need not worry that that his movement alone—when the antilife shell effect encounters no appropriate creatures to hedge out—is somehow sufficient to end the spell once its in effect. Obstacles stop the spell's effect, but they don't end the spell.
Note: Consider also this fine answer that directly addresses the tactics of the NPC in the adventure path, providing valuable insight into the NPC's actions that this answer could not.