How much damage do you take when traversing an area filled with Burning oil in multiple squares?

Question

Suppose there is a creature standing in the centre of a 3x3 grid on which oil has been poured:

+---+---+---+
|   |   |   |
+---+---+---+
|   | C |   |
+---+---+---+
|   |   |   |
+---+---+---+

Note: because the question depends on specific assumptions about the rules, I have highlighted them within the text.

For the purpose of this exercise I assume that the players are able to light all the squares at once (this seems reasonable, for a small enough area [Assertion 1]). The RAW has the following about burning oil:

If lit, the oil burns for 2 rounds and deals 5 fire damage to any creature that enters the area or ends its turn in the area. A creature can take this damage only once per turn.

Accordingly, the creature takes no damage when the fire is first lit [Assertion 2]. But if the creature runs out of the area that they will only take 5 damage because they need to enter a new (single) square with fire in order to escape. This would mean 5 damage in total. [Assertion 3]

Which leads to the question: If the area were bigger (or something/someone used their movement to cross the entire grid (3 squares), would they take 5 damage (using the once per turn limit), or more?

While I am inclined to "more" damage, the (odd) rationale for only taking 5 is that they could linger in a square then move out and only take 5, or they could move into/out/into/out/into/out of a single edge square and only take 5.

Answer 1

A 15x15 area of oil is made up of 9 5x5 areas of fire, each of which can deal damage

The oil flask item states (emphasis mine):

[...] You can also pour a flask of oil on the ground to cover a 5-foot-square area, provided that the surface is level. If lit, the oil burns for 2 rounds and deals 5 fire damage to any creature that enters the area or ends its turn in the area. A creature can take this damage only once per turn.

A single flask of oil will create one 5x5 area of fire which can damage a creature once per turn. In contrast, nine flasks of oil will create nine 5x5 areas of fire each of which can damage a creature once per turn. In your example there simply is no 15x15 area of fire, there are instead 9 5x5 areas, each independent of the rest.

Imagine you run down a narrow hallway, pursued by the baddies, and wish to cover the hallway in oil and then light it. Ordinarily, setting this up would take numerous actions, and thus there is no feasible way to do it in time. If the GM is allowing traps or vats or oil to be used, they should take into account how this affects balance and should rule on how taking damage when walking through such an area works.

Answer 2

I would limit the damage to 5 points.

Bear in mind that oil in this context is generally slow-burning lamp oil, not any kind of modern refined petroleum product. From a game balance perspective, a flask available for one silver piece shouldn't compete in power with magic spells that do fire damage.

Here are the rules for oil flasks from the Player's Handbook:

Oil usually comes in a clay flask that holds 1 pint. As an action, you can splash the oil in this flask onto a creature within 5 feet of you or throw it up to 20 feet, shattering it on impact. Make a ranged attack against a target creature or object, treating the oil as an improvised weapon. On a hit, the target is covered in oil. If the target takes any fire damage before the oil dries (after 1 minute), the target takes an additional 5 fire damage from the burning oil. You can also pour a flask of oil on the ground to cover a 5-foot-square area, provided that the surface is level. If lit, the oil burns for 2 rounds and deals 5 fire damage to any creature that enters the area or ends its turn in the area. A creature can take this damage only once per turn.

The last, bolded sentence is the one we really care about, but the rest of the rules are important for context. Of course, since there's some ambiguity in this critical sentence, it's up to each individual DM to determine how they will resolve that ambiguity in their own game. What follows is my reasoning about how to interpret this language and how I would rule in my own games.

First, it should be noted that covering a creature with oil from one flask and setting them on fire only does 5 fire damage.

Second, if a creature stands in a square of ignited oil for their entire turn, they also take only 5 fire damage.

We can infer from these two points that in general, being exposed to burning oil for a whole turn will do 5 fire damage.

Let's then consider a slightly more involved scenario: a doorway whose space is filled with ignited oil. If a character's turn consists of running through the doorway, grabbing an object on the floor, and then running back out, it's clear from the rules that the character will take only 5 points of fire damage, since "A creature can take this damage only once per turn".

This is also consistent with the idea that a single turn's exposure to burning oil will do 5 fire damage.

In all of the above cases, the scope of "this damage" is clearly confined to meaning "the damage caused by one specific flask of oil". But it's not clear that that's the limit of what "this damage" might mean. "This damage" could be taken to mean "damage from all burning oil sources".

Let's revisit our burning doorway scenario, but have the character exit the room through a second doorway, also filled with ignited oil. How much damage should the character take?

An argument can be made for either 5 fire damage or 10 fire damage. In the latter case, "this damage" is interpreted to mean "fire damage from this specific flask of oil".

Given that there's no other language present to disambiguate "this damage", we can see which interpretation is more consistent with other information we have about how burning oil is intended to behave.

We know that the cases where a creature has the maximum amount of time possible exposed to burning oil in a turn results in 5 fire damage. These are the first two examples considered above, where a creature is either doused in oil and ignited, or where they stand immobile in burning oil for a whole turn.

So what interpretation of the meaning of "a creature can take this damage only once per turn" is most consistent with the other rules that limit whole-turn damage from burning oil to 5 points? To my mind, it seems most consistent to cap the damage to a creature from all burning oil sources to 5 fire damage per turn.

In the other interpretation, where fire damage is limited per oil source, characters are incentivized to act in ways that don't make much sense in context. For example, if a whole corridor is full of burning oil, a character will take less damage if they stand immobile in one square than if they sprint across several squares.

Finally, given the relative ease that characters can obtain burnable oil, limiting the total per-creature per-turn damage seems more in keeping with the intended power level.