If a Circle of the Shepherd Druid is forced out of Wild Shape due to dropping to 0 Hit Points does it activate faithful summons?
Example: the Druid knows he’s going to need a lot of backup for this big fight, so he turns into a Snail and has the fighter step on him to summon four Allosaurus before casting conjure animals to summon more.
The druid's Wild Shape includes this line: "You automatically revert if you fall unconscious, drop to 0 hit points, or die." So, this is something that triggers (among other things) on dropping to 0 hit points.
The Circle of the Shepherd trait includes this line: "If you are reduced to 0 hit points or are incapacitated against your will, you can immediately gain the benefits of conjure animals as if it were cast using a 9th-level spell slot." So, another effect that triggers to 0 hit points.
If they were mutually exclusive, or one prevented the other, simultaneous effects (as explained in Xanathar's Guide to Everything) would come to play, but since they aren't, both effects trigger. You revert to your normal form and you get your summoned buddies.
This works on unconsciousness as well, by the way, since it includes incapacitation, so they both trigger if you're affected by sleep (snails are particularly susceptible to that, I hear) or similar things. Try not to die by non-hp related means, though, because in such a case no Allosaurus buddies for you.
If a Circle of the Shepherd Druid is forced out of Wild Shape due to dropping to 0 Hit Points does it activate faithful summons?
Example: the Druid knows he’s going to need a lot of backup for this big fight, so he turns into a Snail and has the fighter step on him to summon four Allosaurus before casting conjure animals to summon more.
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Let's see it bit by bit.
The druid's Wild Shape includes this line: "You automatically revert if you fall unconscious, drop to 0 hit points, or die."
So, this is something that triggers (among other things) on dropping to 0 hit points.
The Circle of the Shepherd trait includes this line: "If you are reduced to 0 hit points or are incapacitated against your will, you can immediately gain the benefits of conjure animals as if it were cast using a 9th-level spell slot."
So, another effect that triggers to 0 hit points.
If they were mutually exclusive, or one prevented the other, simultaneous effects (as explained in Xanathar's Guide to Everything) would come to play, but since they aren't, both effects trigger. You revert to your normal form and you get your summoned buddies.
This works on unconsciousness as well, by the way, since it includes incapacitation, so they both trigger if you're affected by sleep (snails are particularly susceptible to that, I hear) or similar things. Try not to die by non-hp related means, though, because in such a case no Allosaurus buddies for you.