I am playing a drow Hexblade warlock, and so far, I am enjoying myself a lot. I was trying to decide what spell to gain for my 8th-level Mystic Arcanum (at warlock level 15). I saw the maddening darkness spell, but I am confused a bit.
Does maddening darkness affect everyone in the area, including the one who cast it, even if they can see through it using the Devil's Sight eldritch invocation?
Excerpted, Maddening Darkness has the following effects:
Magical darkness spreads from a point you choose ... Non-magical light, as well as light created by spells of 8th level or lower, can't illuminate the area.
... Whenever a creature starts its turn in the sphere, it must make a Wisdom saving throw, taking 8d8 psychic damage on a failed save, or half as much damage on a successful one.
As the first effect of Maddening Darkness is simply to produce magical darkness, that particular aspect does not affect a Warlock with Devil's Sight, who can see through magical darkness without issue.
However, the second effect does not depend on the darkness, it simply applies to the same area (it's in a separate paragraph from the description of the darkness and doesn't reference the darkness at all, either). Anyone starting their turn in the area takes the psychic damage that the spell deals - seeing through the darkness doesn't change that, and the spell has no exception which makes the caster immune. You could even illuminate the area using a light-causing spell cast at 9th level, but that would not prevent the damage.
Spells only do what they say they do. Here is the relevant text:
Magical darkness spreads from a point you choose within range to fill a 60-foot-radius sphere until the spell ends. The darkness spreads around corners. A creature with darkvision can't see through this darkness. Non-magical light, as well as light created by spells of 8th level or lower, can't illuminate the area.
Shrieks, gibbering, and mad laughter can be heard within the sphere. Whenever a creature starts its turn in the sphere, it must make a Wisdom saving throw, taking 8d8 psychic damage on a failed save, or half as much damage on a successful one.
Nowhere in the text does it state there is a save, or an exclusion. Anyone that starts their turn inside the sphere is affected. Being able to see through it only means you can see what is coming for you.
Also, there is no general concept of turning off "friendly fire" in 5e. You can die by centering a fireball at your feet.
There are a few ways to help if an ally would be caught in the blast:
When you cast a spell that forces other creatures to make a saving throw, you can protect some of those creatures from the spell's full force.
You can create pockets of relative safety within the effects of your evocation spells.
And while they may help with the damage, neither would keep the ally from being surrounded by magical darkness.
