I'm researching the attack spells available to druids. Creeping Doom looks very useful for some situations, but it seems like it doesn't combine well with other spells like Flame Strike and Fire Storm since you'll quickly destroy your own swarms.

Can a druid use Selective Spell to mitigate this by excluding the swarms?


Most probably no

While Swarms "act" as a single creature, they are not "treated" as a single creature, except when the subtype mandates it: HD, HP, initiative, speed, AC and Saves.

As an exception to all these things where the swarm is treated as if it was a single creature, swarms are immune to any spell that targets a specific number of creatures. While I have no proof that this is true, I think this ability is meant to represent the fact that the swarm, despite having a single AC, HP pool, and so on is still made of individuals and affecting a bunch of those will not stop the whole swarm.

While there are no rules specifically about this rare situation, I infer that the swarm should also not be immune to any spell that avoids targeting a specific number of creatures, as a Selective spell does: you have spared a few chosen individuals, which are Diminutive and therefore not big enough to be a threat (the smallest Centipede monster is Tiny), but the bulk of the swarm has been damaged.

  • \$\begingroup\$ Swarms are treated as a single creature for most effects; this answer needs to back up the implied assertion that Selective Spell is an exception. \$\endgroup\$ – KRyan Nov 23 '19 at 19:00
  • \$\begingroup\$ @KRyan do you think the reasoning holds? Of course I'm into Rules-As-Interpreted territory, but I don't think the manuals ever give a RAW answer to this very peculiar occurrence. \$\endgroup\$ – Zachiel Nov 25 '19 at 18:25
  • \$\begingroup\$ I do not think it does, no. I agree that there probably is no more to be found in the rules on the subject, but to my mind anything that considers individual elements of a swarm rather than the singular swarm creature is a mistake. That isn’t necessarily to say that mistakes weren’t made—after all, the swarm subtype wholesale is largely one mistake after another. \$\endgroup\$ – KRyan Nov 25 '19 at 18:49
  • \$\begingroup\$ Well, it's a mistake they started. \$\endgroup\$ – Zachiel Nov 25 '19 at 20:49
  • \$\begingroup\$ My point is that I don’t think this is one particular mistake they did make—I’m just acknowledging that they made many and it wouldn’t be that surprising if they did. But I remain unconvinced by the evidence before me that they actually did. \$\endgroup\$ – KRyan Nov 25 '19 at 21:00

Yes, but probably not the way you are thinking

The selective spell feat allows you:

When casting a selective spell with an area effect and a duration of instantaneous, you can choose a number of targets in the area equal to the ability score modifier used to determine bonus spells of the same type... These targets are excluded from the effects of your spell.

Emphasis added.

A creature with the swarm subtype:

is immune to any spell or effect that targets a specific number of creatures (including single-target spells such as disintegrate), with the exception of mind-affecting effects (charms, compulsions, morale effects, patterns, and phantasms) if the swarm has an Intelligence score and a hive mind.

This means that:

  • A swarm is immune to any spell that targets a specific number of creatures. It can still be targetted by such a spell, but it will suffer no effects of said spell
  • This does not apply if the spell gains the [mind-affecting] descriptor and the swarm gains an intelligence score "and a hive mind". (Centipedes typically lack both, so I will not further address this)
  • It doesn't matter if a spell has multiple effects, some targetting only a limited number of creatures, some targeting everything in an area: any spell (or other effect) that targets for any purpose a specific number of creatures will see none of its effects affect a swarm
  • A selective spell targets a specific number of creatures, even though the base spell it would have been without metamagic feats wouldn't have done so.

And, in conjunction:

  • A selective spell never affects any swarm, whether you want to exclude them or not
  • \$\begingroup\$ I’m not 100% certain this works out here—you choose a number of targets to exclude, which isn’t necessarily the same as choosing a number of targets to include, which means the spell may well still just be an area effect for any creatures not chosen with Selective Spell. \$\endgroup\$ – KRyan Nov 23 '19 at 19:01
  • \$\begingroup\$ I'm unsure what you mean by a difference between targeting creatures to exclude or include. It seems to me that the swarm traits just care about whether a spell targets some creatures, not what it targets those creatures for. I assume you are suggesting there is a chain of logic that links being targetted to be excluded from an effect and technically not being targetted in a sort of non-hearsay hearsay exception sort of way, but if so I am not seeing it. I can approach this instead/in addition in a manner addressing that, I think, but it would be helpful to understand better the objection. \$\endgroup\$ – Please stop being evil Nov 23 '19 at 23:27

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