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.