Potential effects of non-standard creature size in combat (purple worm)

Question

My players are about to finish G3 (Hall of the Fire Giant King) and start D1 (Descent into the Depths of the Earth) - note that I am converting these 1e modules to 5e.

I am rolling the wandering encounters in advance between each section of planned encounters, and their first encounter will be with a purple worm (which they have not faced before, both the specific PC's and likely the players). A single purple worm (CR15) against the party (six PC's of 11th level) is expected to be an 'easy' fight. I would like to add some element to increase the dramatic difficulty, but it doesn't make narrative sense to either have a second worm or a number of smaller 'minion' creatures.

At present, I am planning to have the worm emerge from a side tunnel (actually from a side tunnel inside a sinkhole, so that multiple tunnel mouths are present but none in view of the party as they are crossing a natural stream running along the primary tunnel). I am hopeful that if enough of the party are surprised, or are busy crossing the stream, the worm can emerge unexpectedly and have a fair chance to swallow at least one of the party before action economy takes over and they stun it, then hack it to pieces. I could have the worm begin the encounter by burrowing through the ceiling or floor, guaranteeing surprise and ambushing the party point blank from total cover, but I try to have a more naturalistic atmosphere (at least for wandering encounters). The intelligent drow ambushing the party are preparing specifically for them, which is fine, but it will strain the players' suspension of disbelief to have a purple worm appear right below them in a well-traveled Underdark highway as if it had been waiting for just them this whole time. [I am open to frame challenges that suggest other ways to increase the difficulty of the encounter, though]

I have never run a purple worm in 5e, and its movement, especially movement in tunnels, seems wonky. The worm has a speed of 50', and a burrowing speed of 30'. In order to move through solid rock, the worm uses its tunneler feature, which says:

Tunneler. The worm can burrow through solid rock at half its burrow speed and leaves a 10-foot-diameter tunnel in its wake.

Thus when moving through solid rock, the worm can move at 15' / round (or, I suppose, dash at 30') and leave a 10' wide tunnel behind it - but when moving through a pre-dug 10' wide tunnel...it can't. It can't actually move through the tunnels it has already created, since as a gargantuan creature it occupies a 20' x 20' square (we play on a VTT grid). It could squeeze through a 15' wide tunnel at half movement speed and with disadvantage on attacks, but it is not even permitted to enter a 10' wide space. RAW, it seems like it has to continually re-excavate the tunnels it has already made by moving through them at burrowing speed with its tunneler feature and then leaving a tunnel behind too narrow for it to use.

In order for it to move quickly through its own tunnels and be able to attack in them without disadvantage, I would like to give it the following homebrew feature:

Worm-shaped. As a bonus action, the worm can alter its shape from 20' x 20' to 10' by 40' or vice versa. While in worm-shape, it has a specific head end, a tail end, and it is considered to be the width of a Large creature for the squeezing rule. It may bite only targets within reach of its head end and sting only targets within reach of its tail end.

I am interested in a review of this homebrew feature in terms of how effective it will be at accomplishing my stated design goal (moving quickly in pre-dug tunnels so as to be able to ambush the party, and being able to fight without disadvantage if it is encountered in a tunnel). Good answers could also include examples of tested designs to accomplish the same thing.

Answer 1

5e already has a "standard" language for moving through a small space

Many creatures, especially amorphous ones such as air elementals, have an ability that lets them move through smaller spaces than the standard squeezing rule allows, and it even lets them do so without squeezing:

It can move through a space as narrow as 1 inch wide without squeezing.

At minimum, I would recommend starting with this language, since it represents the "standard" way in 5e to implement the mechanics of a creature that can move through a space that would normally be too small for its size class. So you could give your worm this ability:

Worm-shaped. The worm can move through a space as narrow as 10 feet wide without squeezing.

And for the sake of simplicity, you might just want to leave it at that. The worm's 10-foot reach already ensures that its "melee radius" is 40 feet from end to end, so using this rule gives it melee access to a 40-foot length of tunnel. But if you feel the need, you can add on additional features to this mechanic as necessary. For example (continuing from above):

Worm-shaped. The worm can move through a space as narrow as 10 feet wide without squeezing, by uncoiling its body. While uncoiled, the worm controls a space 10 feet wide and 40 feet long with its head at the front end, and its Bite and Tail Stinger attacks originate from its head and tail, respectively.

However, I'm not sure spelling out all these mechanics is worth the effort. As the DM, you control the monster's actions, so as long as you know where the worm's head and tail are, you can have it use the right attacks on the right targets, or skip attacks that have no suitable target in range. If the worm moves in such a way that its head is not in range to bite anyone, just don't use its bite attack that turn, even if by the RAW mechanics it should be able to. I doubt your players are going to call you out on taking a mechanically sub-optimal turn for the monster in the name of verisimilitude (especially if the players are not familiar with the monster).

Running a purple worm on a VTT

For what it's worth, I have interacted as a player with a large worm-shaped creature on Roll20. It was sentient and we didn't end up fighting it, but it did move around a bunch while talking to us, and the DM implemented this by having independent 10-foot-long "segments" that they moved around the area one at a time, manually arranging them into the shape of the creature. It seemed to work well enough, and it allowed for the creature to assume arbitrary curved shapes, not just a rectangle. I haven't DM'ed on Roll20, so can't say how much work it was for the DM to manage, this, but I imagine that a worm made of 4 large segments probably isn't much more work to manage than 4 independent large creatures. Your players might actually find this more engaging than fighting a featureless 20x20 square, and you get natural origin points for the head- and tail-based attacks.

Does a purple worm actually re-use its own tunnels?

The description of the purple worm in the monster manual contains this interesting passage:

A burrowing purple worm constantly creates new tunnels throughout the Underdark, which are quickly made use of by other creatures as corridors and highways. Because a purple worm rarely returns to its own tunnels, such passageways are a good place to avoid these monsters. Areas rich in prey quickly become interlaced with complex tunnel systems resulting from several worms hunting together.

As you have observed, strict application of RAW indicates that a purple worm cannot re-use its own tunnels. This seems to have a curious resonance with the passage quoted here, so perhaps this was not an oversight. (Or perhaps it was an oversight and they added the lore about not re-using tunnels to paper over it.)

Should a purple worm re-use its own tunnels in combat?

As for re-using tunnels during combat, this is unlikely to be tactically sound. The worm's speed is 50, and you've given it a length of 40, which means that after using its full movement to enter a tunnel, its tail will still only be 10 feet from whatever it is running away from. Of course it could dash to move another 50 feet, but then it makes no attacks on that turn. So it might actually be reasonable to decide that once it has Kool-Aid Manned its way through the wall, the worm simply doesn't use tunnels during combat, but rather coils up into a defensive posture and starts fighting. This saves you from having to solve the problem entirely.

Answer 2

You don't need to homebrew a thing. Gargantuan creatures occupy a 20'x20' square for combat purposes. That doesn't mean they're actually 20'x20'. A halfling occupies a 5'x5' square, but last I checked, they are not actually 5'x5' (aside from the fattest of halflings). As your own link notes (emphasis added):

Space

A creature's space is the area in feet that it effectively controls in combat, not an expression of its physical dimensions. A typical Medium creature isn't 5 feet wide, for example, but it does control a space that wide.

The monster could easily be a mere 10' in diameter and 40' long all the time, it just curls into a fighting posture where it sways across a 20'x20' space, but tunnels in a straightened, narrower form.

Even if you want to play it rules as written (where moving through a space smaller than its dimensions requires squeezing), you can just declare it a 10'x40' monster all the time if you really want; Gargantuan sets a minimum size, but it seems perfectly reasonable to use any set of dimensions that makes sense and occupies an area at least as big as the 20'x20' minimum size for Gargantuan creature (20 x 20 == 400 == 10 x 40 after all).