We just had a scenario where the enemy set up a 'death trap' with arrow slits covering a corridor, allowing them to pop in, shoot, then pop out — all without actually entering the corridor. Which was fine in and of itself, until my cleric ran through the corridor at top speed, with Spirit Guardians up.
The DM and I discussed it, and as far as we can tell the spell doesn't require a line of sight and isn't blocked by walls — just being 'in range' gets you the damage. So I wiped out an entire encounter with one well placed third level spell. In and of itself, that doesn't seem too OP — at level 5/6 a third level spell is a pretty big investment! But it still seems a bit odd that I can do it without even LOS.
Are there any rules I'm missing?
Unless an AoE spell has a specific exception, it doesn't affect creatures through walls. To establish the type of spell Spirit Guardians is, we have the following:
Spirit Guardians has:
so it clearly fits into this category. The important rule here is on page 84 of the Player's Basic Rules (page 80 if you have an older version of the basic rules PDF), under the Areas of Effect section:
So if a creature has total cover from the point of origin, they're not affected by Spirit Guardians.
However, in this case, the targets can be affected if they're standing directly behind the arrow slit.
So if the enemies are behind the arrow slits, they have three-quarters cover rather than total cover, and they can be affected by Spirit Guardians.
There is an additional potential issue with this strategy, but that was covered in this question.