Outrun the Explosion

One of my favorite adventure story tropes is people outrunning something huge and terrifying. Indiana Jones and that raging river in the Temple of Doom, the Catching Fire kids and that deadly mist, and almost any action movie with an explosion in it are perfect examples of this trope at work. Some of my favorite D&D sessions have ended with PCs running through narrow dungeon passages as some hazard like a wave of acid or crumbling ceiling chases right behind them. I’m aware that it’s impossible to outrun an explosion in the real world. So is casting magic missile.

Since I love this trope so much, I’ve created a few rules for how I use them in my games and I thought I’d give them to you! Check them out below.

Outrun Hazards

Outrun hazards are moving bodies of hazardous materials (like acid rivers or poison gas) which travel through enclosed spaces (like dungeons) and bear down on the characters. It helps if you have a map and minis to track the encounter when using outrun hazards.

Map. You’re going to need some sort of gridded or hexed map to help you keep track of the outrun hazard, especially if the encounter takes place in tight, winding tunnels. A map you can draw on is preferable, but if you can’t draw on the map use poker chips, tokens, or dice to help you track the hazard along the map. All of the other factors of your hazard can be tweaked based on the map you pick.

Tokens or Minis. You’ll need these to track the positions of PCs and other creatures on the map as they attempt to outrun the hazard.

There are a few common traits all outrun hazards have.

1. Trigger. All outrun hazards have a trigger which makes them start moving. This could be anything from the lighting of a fuse to the breaking of a dam wall.
2. Point of Origin. All outrun hazards have a point of origin. This point can be as small as a 5-foot square or as large as the entire edge of a map.  This is the space in which the hazard begins its run and should appear once triggered.
3. Height. All outrun hazards have a height which should be noted at the start and can change as the hazards moves. (Note: If you want a direct flow outrun hazard to remain effective in a place with many wide open spaces, it’s fine to give it a height of “infinite” so running through wide spaces won’t suddenly eliminate its effectiveness. Not only that, it makes your life easier since you don’t have to track the height of the hazard. More on that below.)
4. Flow. Flow describes the direction in which the outrun hazard moves. The various types of flow are listed below.
   • Direct. From the point of origin, this hazard moves in one direction as a straight line. When the hazard moves into an area wider than its current width, the hazard’s width changes to match that area’s width. For every 5 feet wider the hazard gets, it loses 1 foot from its height. For instance if a 10-foot-high, 50-foot-wide river of acid moves into a 60-foot-wide hall, it becomes a 8-foot-high, 60-foot-wide river of acid. If an outrun hazard moves into a space so wide it reduces its height to 0 feet, the hazard ceases to move and is not effective in that new space. When the hazard moves into a space narrower than its current width, the hazard’s width changes to match that area’s new width. For every 5 feet narrower the hazard gets, it gains 1 foot to its height. For instance if a 10-foot-high, 50-foot-wide river of acid moves into a 40-foot-wide hall, it becomes a 12-foot-high, 40-foot-wide river of acid. An indoor hazard can only grow to a height which the ceiling allows. (Note: If you want your outrun hazard to remain effective in a place with many wide open spaces, it’s ok to give it a height of “infinite” so running through wide spaces won’t suddenly eliminate its effectiveness. Not only that, it makes your life easier because you don’t have to track the height.) When an outrun hazard with a direct flow runs into a wall it begins moving in a new direction. If there is only one way for it to move after hitting the wall, it moves in the only direction it can. If after hitting the wall it can move in more than one direction, the hazard divides into equal parts and retains its height. So if a 10-foot-high, 50-foot-wide river of acid runs into a wall and can move in two directions, it becomes two rivers of acid each 10 feet high and 25 feet wide flowing in their respective new directions.
   • Explosive. Outrun hazards with explosive flows usually move quickly and dissipate even faster. These hazards move outwards from their point of origin in all directions as a sphere. Essentially they are a sphere with a growing radius. When these hazards hit a wall they stop moving in that direction, but continue moving in all other directions.
   • Infinite. Outrun hazards with an infinite flow usually move slowly, but can get into almost any space and take a long time to dissipate. An infinite flow hazard acts a direct flow hazard, except that it travels in all directions, not just a single direction, and it does not lose or gain height when it enters an area of a new width. It simply continues to move in all directions as its confines allow. Once an infinite flow hazard reaches an open, outdoor area, it stops moving in that direction.
5. Initiative. All outrun hazards an initiative modifier and roll initiative as normal. During its turn an outrun hazard can only move. It cannot take actions or reactions.
6. Speed. The speed at which the outrun hazard moves as described by its flow.
7. Surge. On the hazard’s turn roll a d6. If the result is greater or equal to the hazard’s Surge value, the hazard moves twice its speed this turn.
8. Effect. This the effect the outrun hazard has on a creature when it is within the hazard.
9. Terminal Conditions. Eventually the outrun hazard runs out of steam. This entry describes how this occurs and what the lasting effects of the hazard on its environment are.

Sample Outrun Hazards

Here are a few sample outrun hazards. Feel free to tweak them as needed for your dungeons or campaigns.

Acid River

A bright green river erupts from the wall in a burst of pressure, sizzling the stones and objects in its way.

Trigger. There is a huge lake of acid held behind a dam of adamantine in the Underdark. Configuring levers in the right sequence allows for the dam to be opened and the acid to be released.

Point of Origin. A line 50 feet wide and 5 feet long.

Starting Height. 20 feet

Flow. Direct. The river of acid moves and grows in a direction opposite the lake.

Initiative. +1

Speed. 70 ft.

Surge. 5

Effect. A creature who gets caught by, moves into, or starts its turn in the river takes 22 (4d10) acid damage and must succeed on a DC 15 Strength saving throw or become restrained while in the river and move the with the river as it flows. As an action the creature can repeat this saving throw on its turn to try and escape the river. The creature has disadvantage on this saving throw if the height of the acid river is greater than the creature’s height.

With a successful DC 15 Strength (Athletics) check a creature can swim against the flow of the river.

Terminal Conditions. After 5 minutes, the lake has been emptied and the river stops moving. Any unattended objects in the complex susceptible to acid damage in the path of the river are completely destroyed. Small puddles of acid remain on the floor of the complex where the river flowed. Any creature which falls prone on this path takes 5 (1d10) acid damage.

Fiery Explosion

The glass column of elemental energy explodes in a tremendous, fiery burst.

Trigger. Dealing 100 damage to a huge, reinforced, glass column containing the raw elemental energy of fire.

Point of Origin. The column, which is in a cylinder with a 5-foot radius

Starting Height. 20 feet

Flow. Explosive

Initiative. +3

Speed. 90 ft.

Surge. 6

Effect. A creature who gets caught by, moves into, or starts its turn in the explosion takes 33 (6d10) fire damage and must succeed on a DC 15 Strength saving throw or fall prone.

Terminal Conditions. The explosion last 3 rounds and then ends. Unattended flammable objects within the radius of the explosion are completely destroyed.

Insanity Mist

A cloud of purple mist shaping itself to look several grinning, laughing faces emerges from a vent in the floor and begins filling up the complex.

Trigger. The mind-flayer villain wears an amulet around its neck. Upon its death the amulet releases the mist from the vents.

Point of Origin. The 5-foot square of the vent

Starting Height. 5 feet

Flow. Infinite

Initiative. +0

Speed. 50 ft.

Surge. 5

Effect. A non-aberration creature who gets caught by, moves into, or starts its turn in the mist must succeed on a DC 15 Wisdom saving throw or gain a form of short-term madness. If the creature fails this saving throw within the mists while it has a form of short-term madness, it then also gains a form of long-term madness. If it fails this throw within the mists while it has a form of long-term madness, it gains a form of indefinite madness.

Terminal Conditions. The mists linger indefinitely in the complex and can only be removed by a strong wind blowing throughout the complex for an hour. A strong wind can clear out a single room in the complex in 1d4 minutes provided the room is sealed from the rest of the complex.

Lava River

Lava pour forth from the mouths of the massive stone heads on the walls, creating a river of the stuff headed right for you.

Trigger. An ancient red dragon’s death triggers this hazard in its volcanic lair.

Point of Origin. A line 100 feet wide and 10 feet long

Starting Height. 50 feet

Flow. Direct. The river of lava moves away from the stone heads on the wall.

Initiative. +1

Speed. 60 ft.

Surge. 5

Effect. A creature who gets caught by, moves into, or starts its turn in the river takes 33 (6d10) fire damage and must succeed on a DC 15 Strength saving throw or become restrained while in the river and move the with the river as it flows. As an action the creature can repeat this saving throw on its turn to try and escape the river. The creature has disadvantage on this saving throw if the height of the river is greater than the creature’s height.

With a successful DC 20 Strength (Athletics) check a creature can swim against the flow of the river.

Terminal Conditions. After 5 minutes, the lava stops flowing. Any unattended objects in the complex susceptible to fire damage in the path of the river are completely destroyed. The molten lava remains in the complex though now it is simply a placid lake of the stuff which never cools and is 5 feet deep. Any creature which enters or starts its turn in this lava lake takes 22 (4d10) fire damage.

Raging River

The dam breaks, unleashing a torrent of water upon you.

Trigger. A shoddily made dam holds back a small lake. The dam can be broken open as an action with a successful DC 15 Strength check.

Point of Origin. A line 20 feet wide and 5 feet long

Starting Height. 10 feet

Flow. Direct. The river flows away from the lake.

Initiative. +1

Speed. 70 ft.

Surge. 5

Effect. A creature who gets caught by, moves into, or starts its turn in the river must succeed on a DC 15 Strength saving throw or become restrained while in the river and move the with the river as it flows. As an action the creature can repeat this saving throw on its turn to try and escape the river. The creature has disadvantage on this saving throw if the height of the river is greater than the creature’s height. Whenever the river flows and the creature moves with it, that creature takes 11 (2d10) bludgeoning damage from getting banged against walls and other objects.

With a successful DC 15 Strength (Athletics) check a creature can swim against the flow of the river.

Terminal Conditions. After 1 minute the lake has no more water and the river stops flowing.

Lighting Explosion

The glass column of elemental energy explodes in a tremendous burst of blue lightning.

Trigger. Dealing 100 damage to a huge, reinforced, glass column containing the raw elemental energy of lightning.

Point of Origin. The column, which is in a cylinder with a 5-foot radius

Starting Height. 20 feet

Flow. Explosive

Initiative. +3

Speed. 90 ft.

Surge. 6

Effect. A creature who gets caught by, moves into, or starts its turn in the explosion takes 33 (6d10) lightning damage and must succeed on a DC 15 Constitution saving throw or become restrained for as long as it remains in the explosion. The creature can repeat this saving throw on its turn as an action, ending the restrained condition on a success.

Terminal Conditions. The explosion last 3 rounds and then ends. Unattended flammable objects within the radius of the explosion are completely destroyed.

Poison Mist

The statue of the dragon’s head breathes a sickly green gas into the air and an enormous cloud begins to form and take over the complex.

Trigger. A lever on the wall next to the dragon’s head.

Point of Origin. A 20-foot-radius sphere of gas

Starting Height. 20 feet (see Point of Origin)

Flow. Infinite

Initiative. +0

Speed. 50 feet

Surge. 5

Effect. A creature who gets caught by, moves into, or starts its turn in the mist takes 44 (8d10) poison damage and must succeed on a DC 15 Constitution saving throw or become blinded for 1 minute.

Terminal Conditions. The mist lingers in the complex in 1d10 days. It can only be removed before that by a strong wind blowing throughout the complex for an hour. A strong wind can clear out a single room in the complex in 1d4 minutes provided the room is sealed from the rest of the complex.

Spice It Up

Obviously the examples listed here can be tweaked and modified to fit you specific dungeon needs. One quick way to make these outrun hazards even more exciting is to introduce chase complication tables and extra dash actions from the Dungeon Master’s Guide into the encounter.

PDF

For your convenience I put the rules and sample outrun hazards in a free PDF for you in the link below.

Outrun Hazards

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