I've been working on this one for a while, and I'm happy I've finished it in time for the release of Dune 2!
The Ashworm Dragoon was a prestige class in DnD 3.5, meant as a "mundane" (as in, non-magical) special mount for fighters.
The two main hurdles when writing this class was finding a way to have the ashworm come back to the player in the event of death (most familiar subclasses in the game don't have this problem, as the character has spell slots to heal/resurrect their faithful friend) and balance the burrowing speed of the ashworm (I actually found that the rules are a bit more restrictive than I thought, and I've added a few more to prevent strategies that the players could employ to cheese encounters using their burrowing speed).
Hmm. This is a really interesting subclass! I'm not the best at evaluating pet builds, but I can see that this has all the needed mechanical notes in being able to heal your pet, ensuring they don't break the action economy, and making sure they fulfil that role of being a second body that can be effectively positioned around enemies.
The main issues I can imagine an Ashworm Dragoon running into when played have absolutely nothing to do with this subclass and more to do with flawed game balancing at higher levels, so as far as I'm concerned this is a very well-designed subclass.
It's also particularly good because of the order you get your features in.
HP is most useful from levels 1-4, and you get your big HP-having Ashworm first. The HP calculation on it is a little odd but not problematic.
Unrivalled Duo comes in at level 7, which is the earliest the subclass could reasonably get a fear aura (which is something any martial that wants to be a tank craves. Even if this fear aura has very conditional activation conditions, the ability is so excellent for supporting the team that it's still awesome to have.)
Minor thing about the level 10: You use the example 'if your ashworm is killed' for ways it might be forced to stop burrowing, which contradicts its' minor plot armour from the level 3 thing, so you might want to pick a different example. Anyways the damage boost is nice and helps keep pace with magic weapons, but it's nothing crazy.
Potent Poison would be kind of underwhelming in a more balanced game, since most enemies big enough to warrant a stun at level 15 are going to be packing mighty Con Saves. An adult red dragon (which is no match for a fully rested level 10 party, let alone a level 15 party) has +14 to Con Saves, IIRC. But even much tamer monsters can easily be rocking +9s and +8s, because enemies tend towards higher Con in a futile attempt to let them survive player attacks. But really by this stage then the main flaw of 5th edition DnD is well and truly in effect, so honestly it barely matters what features a fighter gets- at 13th level and beyond, they're just not capable of keeping up with casters. That said this feature IS designed and balanced in accordance with existing fighter subclasses, and I cannot fault you on that design decision. Not at all.
Hunter of the Sands, similar to Potent Poison, isn't a very strong feature since a lot of high level enemies can easily bypass it. But again, that's not something that I'd mark down a fighter subclass for, because you'd have to overhaul Fighter itself in order to mend that problem.
Anyways, I believe that as an overall analysis, I'd give this... 9.1/10
Yeah, I was trying to keep the subclass on the same level as the other fighter subclasses, but I can see how the later levels can feel a bit lame. I've added a few more things (+2 to the Sting saving throw at 15th level, and +1d6 to the ashworm's damage at 18th level), and I've reined back the Terror Cry ability (60 ft -> 30 ft, and it increases to 60 ft at 18th level).
As always, I'm very grateful for your analysis! I always try to imagine scenarios and run the math for my subclasses, but certain features always end up over/underperforming and it's difficult to see it without an external POV. I really appreciate your help.