With the casting of Spirit Guardians, the question has arisen: when does a persistent area of effect spell activate/take effect?
A spell that creates a momentary area of effect - Shatter, Fireball, et cetera - will do damage immediately.
But a spell that creates a persistent area of effect - Blade Barrier, Cloud of Daggers, Healing Spirit, Ravenous Void, Spiritual Guardians, et cetera - is described as taking effect when either:
a) a character first moves into that area on a turn
or
b) a character starts its turn in the area
It turns out that there is a LOT of debate about what that means. I'm using this post to describe my ruling on it: when it applies, and when it doesn't.
There are five possible scenarios/cases that could be interpreted as applicable.
1)
A character is inside the area of effect when the spell is cast.
I've decided that
this does NOT apply. Yes, the character is in the area of the spell, but the character did not move to get there. Since the spell's area of effect appeared around the character, rather than the character moving into the spell's area of effect, it does not apply.
Wizard Jo casts the spell, and Loren is in the area of effect, but Loren does not take damage when the spell was cast on Wizard Jo's turn.
2)
The spell has already been cast, and a character starts its turn in the area of the spell.
Obviously
this applies. So if Wizard Jo casts Cloud of Daggers and Loren is in the area of effect at the start of Loren's turn, then Loren is affected by the spell (with Cloud of Daggers, Loren takes 4d4 piercing damage at the start of their turn).
3)
The spell has already been cast, and the caster moves the area of effect to include a character.
This does NOT apply, either, because the
character being affected is not the one moving. Ranger Pat casts Healing Spirit then moves the Healing Spirit into Loren's space. Loren doesn't get healed then; Loren can be healed at the start of Loren's turn if the Healing Spirit is still there.
4)
The spell has already been cast, and a character outside of the spell moves into the area of effect on their turn.
This applies. Wizard Jo has cast Evard's Black Tentacles, and then on Loren's turn, Loren enters the area of that spell. Loren then has to immediately roll the DEX saving throw or take 3d6 bludgeoning damage and be restrained.
5)
The spell has already been cast, and a character inside the spell's area of effect is moved out of the area of effect on someone else's turn.
This does NOT apply. Wizard Jo has cast Moonbeam, and Loren is caught in the area of effect. Wizard Jo ends their turn. A third character, Druid Jules, takes their turn, casting Thorn Whip on Loren. Loren takes damage, fails their saving throw, and is pulled out of Moonbeam's area of effect. Loren is moving, but not INTO the area of effect. Whether or not Loren is moving OUT of the area of effect, they are already in the area of effect, rather than moving into it.
6)
The spell has already been cast, and a character outside the spell's area of effect is moved into the area of effect on someone else's turn.
This applies. Wizard Jo casts Cloudkill and ends their turn, with Loren safely 5' outside the cloud. Druid Jules casts Thunderwave, Loren fails their save, and Loren takes damage from the Thunderwave...AND gets pushed backwards into the Cloudkill area of effect. Loren immediately has to roll the CON saving throw. Cloudkill affects Loren because they moved INTO the area of effect.
OOC:
(For this last one, note that this differs from attacks of opportunity, which explicitly state that "Forced movement, such as falling or being pushed, does not provoke an opportunity attack.")
