Nasty, huh? This is how solo bosses should be made!
Let's go through it. The players met God-King Orion (which I stole shamelessly from warhammer), who pledged to give them access to a Rune if they pledged themselves to a Hunting Lodge of his tree-city. How does on pledge to a lodge? They agree to go on a hunt, the target of which is chosen by Orion. The players accept, Orion thinks awhile, and he decides on what he wants. "On another world lies worms of titanic size; hunts worthy of my Glade Riders! One's caught my eye; Albino, with the breath of a dragon, and scaled! My Riders will guide you through the Glade, and drive the creature towards you." This plays out, the players make it to this new world (which they didn't at first understand, was not a plane, but another Material World), and look about. Pale red skies and sand lie above whatever ravine or canyon they've landed in, and the Riders set out to drive the worm towards them. Important note; the players knew the Riders were not to assist them in the event things went sideways.
This sort of encounter (the players are specifically contracted to fight it) is a great opportunity to be nastier than usual.
So, let's evaluate what this guy can do.
The big threat to players is the Cold Breath. It's based on a Con save and deals a slightly less common resistance. The Gusting Breath is functionally more deadly to the players. It's based on a slightly better save for players (though not for mine in particular), but deals a far less common resistance (physical damage) and has the potential to blind the players besides. That being said, it can only be used after using the Inhaling Breath. This of course means using its action a whole round beforehand, which minimizes its damage somewhat. Not by too much, though. The Inhaling Breath is actually quite useful on its own; bringing creatures closer to its lovely whirlwind of death is frightening on its own, particularly if you start using the Bull Rush legendary action.
Let's go to the Molting Shriek ability. I detected pretty early on the players were going to attempt a stunlock (and why not, there's only 1 creature). Once we got to a point in the fight that the players discovered holding back might get one or more of them killed, the creature's hp started dropping far more rapidly than before. I decided to give it a reaction (homebrew monster, I can do what I waaaaaaaant). I didn't know the specifics of the reaction beyond 2 things; it dropped his AC by 2, and the players had to make a con save or become stunned. I remember describing it as rearing back in pain, inhaling to emit the shriek so strongly that some of the scales and chitin they'd been striking began to slough off. If I recall correctly, this was roughly around where the creature was at half hp. Half HP is generally a goof flux point to intensify a battle, whether the creature gets stronger, weaker, or whatever else have you. I used this ability a second time however, when he was at something like 20 hp. I didn't have to include a justification for using the ability multiple times, of course. What I invent in the moment isn't always necessarily useful to anyone reading. Nevertheless, I thought I might be able to come up with a clever escalation mechanic, and lo! I did!
You might be wondering "Why doesn't the worm just spam that ability, use legendary actions and breath weapons to kill the pcs, and bounce?" 2 reasons, one of which is tactical, and one of which is not. The tactical reason is spamming Molting Shriek grants diminishing returns. The chances that all of your players will fail the save (even as hefty as 16 Con) is low. The chance they'll all fail it multiple times? Virtually none. So your monster is very rapidly decreasing his armor class (which normally remains static). 5e math is adjusted for players hitting something like 60% of the time. That might be lower when it comes to high AC creatures like this. If the monster spams this ability, he'll be hit with far more attacks and be far less able to defend himself. His HP will begin to spiral downwards; the difference between HP retention in the fight between my players and this creature before and after he used Molting Shriek was very, VERY distinct. That's just going from 20 to 18. Imagine going down to something like 12! You might very well kill a player, don't get me wrong. You're probably not going to get much more than that, though.
I saved the narrative reason for last because I wanted a well thought out, mechanical explanation for the inevitable complaint over things people haven't playtested being "OP". Narrative reasons are nice, but people will glance over them if they dislike the conclusion. There's a very simple reason this creature doesn't do take "tactically completely optimal" option: it has an intelligence of 6. 6! There are no tactics for that creature. It's not going for the spellcaster first. Stop it. Stop typ-no, stop. If you wanna play the half dragon sandworm like it houses the trapped consciousness of Alexander the Great, you do you man.
Don't drag me down.
One last thing; the death burst! This was I think lifted directly from the 3.5 supplement. Very simple ability; the players kill the creature, and the creature detonates and hurts them. This can actually be important in 2 contexts. For one thing, if other creatures are on the way, how many resources (HP, spells) the players have left is actually quite important. Next, if any players are unconscious (particularly more than one), stay in initiative (which I did). The players probably won't lose anyone to failed death saves outside of combat. The possibility remains, however, and who has what spells to revive someone actually matters (at the very least, a non healer might need to spend some health potions to ensure the survival of a close downed player).
Lemme know what you think guys!