Homebrew Rules & Modifications

The Famous Exploding Rolling Boulder Trap (D&D)

Here’s the best trap I’ve ever created for a Dungeons and Dragons game. The next time your PCs find themselves in a cave or mine filled with Tucker’s Kobolds (or the normal variety), surprise them with this.  It’s appropriate for PCs of any level; just adjust the damage and DCs of the trap accordingly. 

Not Your Ordinary Rolling Boulder Trap

This trap plays on the traditional rolling boulder trap made famous by Indiana Jones, but subverts its design, making the typical response to such a trap the worst possible response to this one.  This is not unfair metagame trickery, but rather the purposeful design of the trap’s Kobold designers, who understand adventurers’ expectations and use them against them.

The “boulder” that rolls down a hallway threatening to crush the PCs in this scenario is in fact a large pinata-like balloon of explosive gas shaped like a boulder .  The Kobolds’ goal is to shoot the boulder using their fire arrows when the maximum number of PCs are within the blast radius of the fireball this creates. To prevent PCs from simply running away, the Kobolds have devised another devious trick--alcoves carved into the sides of the hallway that trick unwary PCs into thinking they can dodge out of the way of the boulder’s path.  But the alcoves are not merely mechanisms to corral the maximum number of PCs into the radius of an exploding fireball--they are pit traps lined with poisoned spikes!

The Setup

The PCs enter a narrow hallway that curves to the left, then slopes steeply upward in a straight line (from bottom left of diagram).  At the bottom of the slope, narrow tunnels only wide enough for a kobold to squeeze through cut into the cavern walls on both sides of a portcullis. The portcullis can be raised or lowered by a chain mechanism on each side.  It takes 2 rounds to raise the portcullis, and one round to lower it. The PCs encounter no threat at this point and are able to safely raise the portcullis and continue up the hallway.

Soon the PCs notice 5’x5’ alcoves carved into the walls on the sides of the tunnels.  On the back wall of each alcove is carved the face of a dragon with Kobold-esque features.  The first two alcoves are NOT trapped. The Kobolds purposely designed the hallway knowing that adventurers frequently check for traps but are unlikely to check every single alcove after the first.

If the PCs investigate the hallway sloping upwards, the wall looks largely unscratched except for a few marks where it has been clipped by arrows or spears.  The walls also have large burn marks in several places. These descriptions give the players a chance to guess the nature of the trap--a rolling boulder trap should damage the walls, and the burn marks should be suspicious--so the DM should make this information clear to the players.

Springing the Trap

The Kobolds’ goal is to spring the trap when most of the party is adjacent to the trapped alcoves. In practice, the timing and effectiveness of their ambush will vary based on how the PCs scout ahead and how clumped together they are in the passage.  

When the Kobolds spring the trap, several things happen simultaneously:

    1. The boulder begins to roll down the hallway.  [Kobolds around the corner at the top of the hallway initiated its movement by kicking away a stopper that had prevented it from moving.]
    2. Two Kobolds with bows and flaming arrows appear at the top of the hallway, with more Kobolds’ wielding flaming arrows coming in behind them. The kobolds have ¾ cover until they act due to the boulder between them and the PCs.
    3. A kobold pops out of the tunnel behind the PCs and begins to lower the Portcullis.

The PCs Respond

Now it is the PCs’ turn to respond. The DM should tell the PCs they only have time to do ONE thing only before the boulder reaches them.  Effectively, they have one action, but no movement or bonus action. This gives the PCs enough time, for example, to move into an alcove or shoot the kobolds with a bow, but not both.

Key Point: The DM should describe how the PCs have no time to consult with each other and should ask all PCs to state what they plan to do simultaneously before resolving their actions. This prevents PCs from choosing not to jump into the alcoves after seeing what happens to other PCs.

Resolving the Trap

If any PCs jump into a trapped alcove, they must immediately make a Dexterity saving throw.  If they fail, they are unable to cling to the ledge and fall into the pit trap, where they land on poison-coated spikes and take piercing damage (see table below for all damage rolls and save DCs). Anyone taking piercing damage must also make a Constitution saving throw, taking full poison damage on a failed save or half damage on a successful one. Because the pit trap is only five feet deep, anyone who falls in may still be in range of if the fireball created if the boulder detonates.

The boulder has an AC of 8, 20 hit points and vulnerability to slashing weapons.  If it is reduced to 0 hit points, gas leaks out of the boulder and it collapses before reaching the PCs.  If the boulder takes any fire damage, it explodes immediately, damaging all creatures within a 20 ft radius.  The best scenario for the PCs is to light the boulder on fire themselves, causing it to explode and kill all nearby kobolds.

If the PCs end their response without destroying or otherwise impeding the movement of the boulder, it rolls until it reaches the point where the most PCs are within a 20 ft radius of it.  The two kobolds then attempt to shoot the boulder, and if a fire arrow hits it, the boulder explodes, dealing fire damage to all creatures within a 20 ft radius. If the Kobolds successfully pull off the trap and sense weakness on the PCs’ part, they press their advantage and attack the PCs.  Otherwise they retreat further into the cave to prepare their next traps.

Setting Saving Throw DCs and Damage

Each DM should determine the DC of the trap’s saving throws and the damage it inflicts.  Page 121 of the Dungeon Master’s Guide sets suggested guidelines for trap damage based on character level and the severity level of the trap.  My suggestion is the DM set the damage of the trap so that full damage from both the fireball, spikes, and poison would reduce the average PC from full to zero hit points. But it is up to each DM how challenging or deadly she wants to make this trap.

And that's it!  If the PCs are careful, smart and lucky, they may avoid the trap completely and feel like savy cutters.  If they aren't, well, that will be fun too!

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