My party is about to enter a room full of enemies so I cast Spirit Guardians on myself designating my party as friendly. Our druid casts fog cloud or our warlock casts magical darkness in the room then I proceed to move through the fog or darkness using dodge as my action hoping to damage the enemy with the spirits.
Will the spirits be able to ignore the fog or darkness since they are spirits or will it hinder their attacks? If it does hinder their attacks, how so?
Page 278 PHB:
"An affected creature's speed is halved in the area, and when the creature enters the area for the first time on a turn or starts its turn there..."
The spell only requires the affected creatures to be in the area, it says nothing of the creatures having to be visible. However, choosing which creatures aren't affected does required you to see them, so make sure to cast this before your party cast fog cloud.
The spirit part is simply "flavor" or "fluff" to the spell, its just lets you paint a picture in your head of how the spells looks like.
The mechanical part is how the spell interacts with the rules which in this case is the 15ft radius Wisdom save. So the spirits don't really "see" anything in a mechanical sense, they're just a thematic effect for the spell to reach its in-game effect.
The spirits are part of a spell's effect and the spell says nothing about them being affected by any sort of environmental hazard. They could move through a hurricane, lava, water, fog, darkness, etc. and would still function exactly as the spell says they do.
The spirits also aren't monsters, as defined on page 4 of the MM, and don't have stat blocks so mechanically they have no concept of sight or hearing or any other senses so there's no need to worry about whether they can see in an obscured environment.
Spells that are affected by the environment specifically says so in their description. See Fog Cloud:
It lasts for the duration or until a wind of moderate or greater speed (at least 10 miles per hour) disperses it (PHB, pg.243).
