You create a wall of fire on a solid surface within range. You can make the wall up to 60 feet long, 20 feet high, and 1 foot thick, or a ringed wall up to 20 feet in diameter, 20 feet high, and 1 foot thick. The wall is opaque and lasts for the duration.
When the wall appears, each creature within its area must make a Dexterity saving throw. On a failed save, a creature takes 5d8 fire damage, or half as much damage on a successful save.
One side of the wall, selected by you when you cast this spell, deals 5d8 fire damage to each creature that ends its turn within 10 feet of that side or inside the wall. A creature takes the same damage when it enters the wall for the first time on a turn or ends its turn there. The other side of the wall deals no damage.
At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, the damage increases by 1d8 for each slot level above 4th.
* - (a small piece of phosphorus)
Of course they would because you would still have to pass from the heat side TO the cool side, so you're still going to take damage! No Matter what side you are on if you PASS THROUGH THE WALL, you are going to take damage!
If you START your turn in the wall and then move out of it, you're not entering it and you aren't ending your turn in it.
"A creature takes the same damage when it enters the wall for the first time on a turn or ends its turn there."
Also, you can spend your turn going back & forth through the wall and only take damage once.
I did exactly this in a campaign recently. Our party was on a wall defending a besieged town and when the besiegers reached the base of the wall i cast wall of fire horizontally along the wall facing downward so any enemies on the ground couldn't get within 20ft of the town wall without getting cooked.
I just realised: if you make it a ringed wall and make the flames point inwards, you've just created a furnace.
Anyone near 10 ft. of the wall will take damage. The diameter is 20 ft, and what makes the radius from the center to the wall? 10 ft. Torture your enemies with 5d8 damage every turn :)
Can fire set things on fire? 🤔
Or they can just take the damage once and walk out of it vs being tortured taking 5d8 every turn
I don't think spells should be assumed to do things (like Wall of Fire settting objects on fire, destroying arrows that are fired through the wall, etc) beyond what is specified in the description or if there is something else in the rules that states that something happens to those things.
Not meaning to ignore DM's discretion, but magic is already powerful enough without giving it more things to do.
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