

The worm should effectively burst up out of the ground from nowhere, get something to eat and retreat underground. If you want to run it, it could be a pretty terrifying and deadly encounter. After all there's no reason for it to stick around if it's had a meal. There is certainly a significant possibility of the purple worm eating someone, so if it's something you want to avoid, I'd say substitute out a different monster or have it pop out, eat a horse (or camel) and run away. It also had a 'save or die' tail poison in 1e (because 1e loved save or die), but wouldn't be able to use it against a party.

Being swallowed did automatically kill you after six rounds, but the current damage output looks like it would kill most characters at that level in 2-3 rounds.

The old 1e purple worm wasn't nearly as dangerous (especially the 6-8 size party assumed in 1e) - it had a straight 15 d8 hit dice instead of the d20 with con bonus it gets now, so something like 75 hp vs 250, and a much lower damage output. I wouldn't have a problem with giving the players a really tough fight (especially since this would happen when they have no time pressure), but I wouldn't want to just wipe them out with a hopeless fight. In 5e I haven't played much at higher levels or Dmed yet, so I don't have a good feel for how these things work, so I might just be underestimating just how much higher level characters can dish out. The first Module is for levels 5-7, and the 'typical' 5e party size is 3-5, and a purple worm is a CR15 monster, which makes it seem like this a TPK waiting to happen to me. Most of the conversion makes sense to me, but one thing that seemed odd is that they did a straight 'use the 5e monster stats' for purple worm encounters. I recently picked up 5e conversion PDFs for the classic Desert of Desolation modules.
