How does the Antilife Shell spell work?

Question

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 6

CASTING

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 yes

DESCRIPTION

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.

Answer 1

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

  If an abjuration creates a barrier that keeps certain types of creatures at bay, that barrier cannot be used to push away those creatures. If you force the barrier against such a creature, you feel a discernible pressure against the barrier. If you continue to apply pressure, you end the spell.

  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

  A creature or object brought into being or transported to your location by a conjuration spell cannot appear inside another creature or object, nor can it appear floating in an empty space. It must arrive in an open location on a surface capable of supporting it.

  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.

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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.

Answer 2

Most of those questions are answered in the spell's description.

Does the barrier immediately fail, or does the caster have the option not to force the barrier into the occupied squares?

The barrier is mobile, as the effect is centered on you. If your character moves towards an enemy that is affected by the barrier (undeads and constructs are not, for instance), that counts as "forcing" the barrier, and it will collapse it, ending the spell.

If the caster backs up, does the barrier prevent enemies from the closing the distance a second time?

Yes, that is exactly the most used tactic with that spell. Enemies cannot enter the barrier (but are free to leave). There is no saving throw, so if you cast the spell and move away, enemies cannot walk in. For enemies with spell resistance, they have to make a check every time they walk into the barrier.

The casting time of 1 round is what prevents this from being abusive and overpower (like Emergency Force Sphere is). If you are facing a caster that you identify that is about to cast this spell, you want to interrupt or dispel it, or some members of your group are locked out of combat. And forcing a raging barbarian out of melee combat is exactly the best use of this spell.

About published encounters, sometimes the published tactics are wrong, the person designing the encounters isn't always the same that designed the map. What I see happening quite often is an enemy using an ability that they are not supposed to use that way, because the author of the encounter (not being a game developer per se) does not understand the game rules as well as they should, resulting in tactics that are impossible to actually work.

If the GM plays their NPCs in a smart way, they can usually come up with better tactics than those listed in the statblocks. But keep in mind that those are suggestions so you are not lost about how to proceed in an encounter.

Regarding that specific encounter from Kingmaker, which I did play but not all the way up to the final boss, spoilers ahead:

She can move to other rooms, use that. The NPC knows the PCs (all the way from book 2 or 3 if memory serves me right), so you are advised to also adjust her tactics against them. Also note that her pocket dimension blocks teleportation of all kinds, so they cannot easily get around her. If she knows there is a good archer in the group, you can swap one of her spells for Fickle Winds to get some protection against arrows. She also has enough high-level spell slots to be able to swap one of them by a summoning spell to help her out. She also knows everything that is happening inside her castle via scrying, don't let the players surprise her. She should be well prepared against them by the time they reach her. Finally, do not forget that she is a Nymph, so don't forget her Blinding Beauty ability. Effectively, anyone within 30 feet who attacks/targets her without averting their gaze needs to do a DC 29 Will save every round or become blind. If they avert their gaze, it's a 50% miss-chance for them.