So if the player succeeds on the DC 19 saving throw for the Bonus Action Swallow is the player still Grappled and Restrained? And if so, can the purple worm in it's next round attack and grapple another creature? Must he use it's bonus action before the attack and reattempt to swallow or let go of the player before attempting to attack again? 

Also, the purple worm can now:
  1. Bite and Grapple a player
  2. use Tail Stinger with advantage on the grappled and restrained player
  3. attempt to swallow the player

Edit - When reading the grappled condition it seems the purple worm needs to either successfully swallow whatever it's grappled or let go of it before even attacking again with it's bite.

One Grapple per Hand. A creature must have a hand free to grapple another creature. Some stat blocks and game effects allow a creature to grapple using a tentacle, a maw, or another body part. Whatever part a grappler uses, it can grapple only one creature at a time with that part, and the grappler can’t use that part to target another creature unless it ends the grapple.

Quote from marcjohne >>

So if the player succeeds on the DC 19 saving throw for the Bonus Action Swallow is the player still Grappled and Restrained? And if so, can the purple worm in it's next round attack and grapple another creature? Must he use it's bonus action before the attack and reattempt to swallow or let go of the player before attempting to attack again? 

Also, the purple worm can now:
  1. Bite and Grapple a player
  2. use Tail Stinger with advantage on the grappled and restrained player
  3. attempt to swallow the player

 

Edit - When reading the grappled condition it seems the purple worm needs to either successfully swallow whatever it's grappled or let go of it before even attacking again with it's bite.

One Grapple per Hand. A creature must have a hand free to grapple another creature. Some stat blocks and game effects allow a creature to grapple using a tentacle, a maw, or another body part. Whatever part a grappler uses, it can grapple only one creature at a time with that part, and the grappler can’t use that part to target another creature unless it ends the grapple.

You do not need to leave the stat block to resolve this question. The Bite attack reads:

• "If the target is a Large or smaller creature, it has the Grappled condition (escape DC 19), and it has the Restrained condition until the grapple ends."

The Swallow bonus action does not have a part labelled "Success," and the text after "Failure" applies only on a failed saving throw. So, if a target succeeds on its saving throw, then it is as if the Swallow bonus action never happened. The creature therefore still has the Grappled and Restrained conditions as described under the Bite attack. There is nothing in the Bite attack that prevents the Worm from making a Bite attack while a creature is Grappled and Restrained as a result of a prior Bite attack.

Adding the person for scale into the artwork was a great idea, it seems a lot more terrifying now.

Man, with its -3 CHA save, this thing is begging to be Banished.

No legendary or magic resistances don't give it much of a chance vs. Polymorph, either.

Even with only a +4 to Wisdom saves, Charm Monster could be pretty effective. I would certainly prefer to Charm this than send it away!

Quote from sethtucker1 >>

Man, with its -3 CHA save, this thing is begging to be Banished.

No legendary or magic resistances don't give it much of a chance vs. Polymorph, either.

Both offer temporary reprieve (Polymorph more so than Banishment). Assuming there's actually a need to defeat the thing rather than just run, those at least give the party time for additional preparation. However, once those spells end...LOL

Also, any DM worth their weight in gold would telegraph that something big was coming but also set things up such that you've had to deplete resources before this thing rears its head. A DM could still fairly throw this thing at a party of 5 9th-level adventurers, which means that while they could conceivably have access to these spells and some a few slots to use them, they should have already used a bunch of those slots up.

Quote from The_Clark_Side >>

Even with only a +4 to Wisdom saves, Charm Monster could be pretty effective. I would certainly prefer to Charm this than send it away!

If there was a way for you to get that spell off before you were actually fighting it, then you'd have your best chance. Depending on how the DM determined the start of combat, even the intent of casting a hostile spell could require that initiative be rolled such that this would be your action on your first turn, in which case the worm would have advantage on that saving throw, which at least increases its chances a bit.

The bigger challenge would be actually taking advantage of a successful casting of the spell in a meaningful way. Once charmed, the worm isn't going to attack you, but all of your allies (and let's say the theoretical townsfolk you're trying to save LOL) would still be very much on the menu, so unless you had some way to actually communicate with it you would have just saved yourself from being dinner. LOL