As far as traps go, the Dungeon Delver feat provides these benefits (PHB, p. 166, 2nd and 3rd bullets) :

  • You have advantage on saving throws made to avoid or resist traps.
  • You have resistance to the damage dealt by traps.

Does the feat still give resistance to the traps damage once they're revealed? Here are a few specific cases :

  • An open spiked pit is found. Does the delver still get resistance to the trap if he knowingly tries to jump over it?
  • A Wall of fire is triggered from a Glyph of Warding spell. Does the delver still get resistance to the fire if, several seconds after the spell is triggered, he knowingly passes through the wall?

The source of the hesitation is the wording of the feat's description :

Alert to the hidden traps and secret doors found in many dungeons, you gain the following benefits [...] (emphasis mine)

In other words, one might think that once a trap is revealed, and it's effects known, it's not a trap anymore, but a mere feature of the terrain, thus restraining the reach of the feat.

As a player, one would like to know so before choosing the feat; as a DM, another might want to know how to deal with delvers hurling themselves in harm's way saying: "Don't worry! That 60 foot deep pit was once meant as a trap... I should get only half of that falling damage!"


What is a trap?

It looks like the crux of your problem is that "trap" is hard to define, particularly with regards to knowledge and time.

After a bit of searching, I found only one location where a trap is actually defined. That's in the Find Traps spell on PHB 241:

A trap, for the purpose of this spell, includes anything that would inflict a sudden or unexpected effect you consider harmful or undesirable, which was specifically intended as such by its creator.

Of course, this is explicitly "for the purpose of [that] spell," so the utility is limited, but it's also the only place where a clear definition for a trap is given.

Based on this definition, a trap is either sudden or unexpected. This means that both of your examples are not traps. They are not sudden, because they have existed for long enough that the PCs can perceive them, and they are not unexpected, because the PCs are aware of them.

Also, if you were to extend the definition of a trap to anything that results from a trap, then this feat could potentially grant resistance to everything in an encounter. For example, a mimic is arguably a trap. Does a dungeon delver get resistance to the mimic throughout the entire fight?


There is a lot to cover in this, so I'll try to break it down.

Short answer; it depends on the trap/DM rules

Start with the description of the "Find Trap" spell

A trap, for the purpose of this spell, includes anything that would inflict a sudden or unexpected effect you consider harmful or undesirable, which was specifically intended as such by its creator.

Traps are generally considered sudden or unexpected; a burst of acid, a falling rock, darts, etc. Traps also have a multitude of functions: Setback, inconvenient, dangerous, deadly (all terms used in the SRD). Here is some more it says about traps:

A trap’s description specifies what happens when it is triggered.

The attack bonus of a trap, the save DC to resist its effects, and the damage it deals can vary depending on the trap’s severity.

Since this question deals with halving the damage of a trap, it would reason that it should be considered an attack. Attacks have well defined rules as far as the initial damage and any residual effects. Traps should also.

In the description of "Collapsing roof" the SRD states:

When the trap is triggered, the unstable ceiling collapses. Any creature in the area beneath the unstable section must succeed on a DC 15 Dexterity saving throw, taking 22 (4d10) bludgeoning damage on a failed save, or half as much damage on a successful one. Once the trap is triggered, the floor of the area is filled with rubble and becomes difficult terrain.

The ceiling is fallen and cannot hurt they player afterwards (other than a stubbed toe) and reduced movement.

However, on pg 196 of the SRD it goes on to talk about "Complex Traps".

Complex traps work like standard traps, except once activated they execute a series of actions each round. A complex trap turns the process of dealing with a trap into something more like a combat encounter.

This could be something in Rube Goldberg fashion with a chain of events, or a long process like filling a room with water or walls closing in. These are still "traps" but nothing sudden about them. Instead they are the catalyst of something else. In cases like this, and the Wall of Fire example, the "trap" is sprung but mechanism causes no damage. The Wall of Fire spell is the aftermath and not a trap.

So pushing the button that causes the toxic gas to spray can be saved for half. The fact that the gas also has an orc pheromone causing all the boys to come to the yard, you're on your own.


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