Casting wall of fire in a T-shaped corridor

Question

In D&D 5e, is casting wall of fire like this allowed, or would I need to have line of sight for the entire corridor? Assume my wizard knows that the corridor extends both ways and how long it is.

Answer 1

This needs DM's approval.

Spell targeting rules prevent you from targeting a point/creature behind total cover :

To target something, you must have a clear path to it, so it can't be behind total cover. If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

So, you can target any point/creature in the intersection, but you can't target a point/creature in the unseen corridors (not because you don't see them - but because there's no clear path.)

Fire bolt is useless here, even if you know someone set an ambush.

Some Area of effect spells can affect creatures in the corridors, as long as there's a clear path between the creature and the spell's targeted point of origin :

A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area. To block one of these imaginary lines, an obstruction must provide total cover.

Fireball can be useful, if the ambushers are close enough : just target the intersection, and the flames will expand from there.

The wall of fire issue

The spell's description lacks reference both to "standard" spell shapes, and to a proper point of origin. Which leaves it up to the DM :

1. He may decide the straight wall uses the "line" template - then you need clear path to one end of it. You can only affect one of the two corridors.
2. He may decide the straight wall derives from the "cube" template - then you need clear path to a point located anywhere on a face. You can affect both corridors.
3. He may decide you need clear path to every inch of the wall. He's the DM, after all. You can affect only the intersection.
4. He may decide you are not targeting anything - and allow you to cast this even without any clear path or point of origin. (not likely the designer's intent). You can affect both corridors.

Option (4) may raise even more questions if the corridor is shorter than the intended wall's length. For (1) and (2), total cover from point of origin would prevent the wall from continuing after the corridor's end : but what happens when there is no point of origin... isn't described in the rules, and lies again in DM's hands.

Answer 2

You do not need line of sight at all

Spells will tell you the requirements to cast them. All spells require (unless indicated otherwise), a clear path to the target, but line of sight is not a general requirement. In other words, spells only require line of sight if they specifically say so.

Here are the targeting requirements for wall of fire:

You create a wall of fire on a solid surface within range. 

Wall of fire does not have any stipulation that you must be able to see the target, therefore it is not a requirement. Just be sure that you are casting within the 120 range.

To be clear, you don't need to see any part of the place where you cast the wall. As long as you have line of effect, you can simply do it. It also does not matter if the caster knows if and/or how long the hall is for the purposes of casting the spell.

Does wall of fire even need a clear path? DM decision

This brings us to a surprisingly tricky question of if wall of fire even needs a clear path to the target. The only thing wall of fire says is:

You create a wall of fire on a solid surface within range.

Unlike many spells, this one does not say what, if anything, it is targeting and also doesn't have any of the standard area of effect shapes which would give us an idea of the point of origin. It is also interesting to note that all of the wall-type spells are written like this.

Regardless, there is no clear answer given to us by RAW or even by RAI and thus the DM must decide how they want to handle the issue. If they rule that a clear path is needed then you can only affect whatever is clear of cover from whatever point they decide is the origin. If not, then your spell works the way you have said.

For more discussion Bash's answer talks a bit more about the options available to a DM here.

Answer 3

Yes you can do this

The description for Wall of fire only lists a range and makes no mention of a line of sight requirement:

... RANGE/AREA 120 ft ... 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 ...

Additionally, the wall can be up to 60 feet so even if the corridor was unfamiliar to the spellcaster and only extended 45 feet, the wall would size itself to fill the space. Do note however that the wall must be created within the stated 120 foot range.