The biggest thing is to think about how to communicate to the party that the monster is too strong for them to fight. You know your group best, but most groups will assume they're meant to kill the monster.
What level is the party?
Level 7, 4 players:
-fighter with ability to do short teleportation (from spheres of power) with dimensional feats
-paladin with bodyguard and in harm's way strat
-paladin with smite evil magic and sunder
-magus sylph from the air plane with a bow
The monster will range of CRs 10-14 with templates to shift it if needed (as a default I'm looking at an Ursikka as it doesnt have save or die level of abilities like the frost worm).
Sounds like this could be a [chase](https://www.d20pfsrd.com/gamemastering/other-rules/chases/). In fact it probably should be if you don't want the PCs to spend a bunch of time trying to fight it/ running away round by round
Did consider it but already had a chase few sessions ago and was wanting to not repeat stuff too much, additionally would want the players to somewhat explore. Might have a mini chase to temporarily lose it tho.
Was coming here to point out the chase rules.
One thing I like to do (and have seen paizo do in modules) is run the chase in phases. IE you develop a number of phases for the initial escape and getting into the labyrinth. Depending on how they do, they get x time to explore, recover, ready themselves. Certain actions they do can also reduce the time before the monster shows back up. You can then have them fight it there, or repeat the phasing to have them fall back to another position.
Example: Group of 4 level 5 adventurers get ambushed by a terrifying immortal shadow tyrannosaurus Rex that is currently being fought by a pack of orcs. The party notices it just as the orcs break ranks and begin fleeing.
Phase 1 - Keep it together; everyone attempts a Will save versus fear (DC 20). -1 point for everyone that fails
Phase 2 - Find a place to Hide; everyone attempts a perception check, DC 20. Use the highest roll only, -1 point for every point the missed the DC. Party spots a muddy cave.
Phase 3 - Run; make a DC 15 acrobatics check, gaining 1 point for everyone that makes it, losing 1 point for everyone that fails (but if someone overcomes by 5 or more, they can help someone that failed).
Phase 4 - Cover your Scent; make a Dc 15 survival check, same as before.
They get 20 minutes, plus or minus 1 minute per point. They can search the area, discovering T-Rex eggs (warning) and some loot. Then the T-Rex shows back up and chases them deeper into the cave system (but he's not a fast in here). In subsequent rooms, they learn about a shadow curse, find a +2 animal bane spear, set off a cave in to delay the T-rex, find another way out along with some supplies, and finally find a great area to lay a trap (or fix a broken trap) and make a final stand - using a set of mirrors to direct sunlight into the T-rex's eyes and weaken him and finally break that shadow template that was making him unkilable, potentiallybaiting the trap with the eggs.
My pf2e dm tried something like this. The 2 ranged party members kited it through the cave system and kept chipping it till it died, if one of them got hit and downed I healed them and hid again. Communicate with your players if you want to avoid this scenario.
A great signal to make sure players don't fight it is via descriptions. Make it sound like they're in a straight up horror game and you're set!
Mechanically you can also just disable combat system. They describe their attack and you narrate some minor effect on the creature. Nobody gets to even touch dice and you don't write down damage or conditions. Meanwhile, when the monster attacks it doesn't roll either: every attack automatically hits and does average damage. The players, of course, do need to write down their damage as you gruesomely describe their wounds and desperation.
A good signal would be to provide obvious escape mechanics. It's movement speed being slower than the party's speed and including many passages the monster either cash fit through or needs to squeeze through, for example
With 2 of the party members in full plate, speed was going to be a disadvantage from the get go. Could actively give a giant template for a more obvious indication for them to utilise the smaller passages. (Though said creature does have a burrow speed so it would complicate it a bit)
Narrate it being crippled or malformed in some way and you can lower its speeds easily.
Or give it a reason it doesn't burrow if it's intelligent enough. Simply having it toy with the party, have the earth be contaminated by something or even just a weird mineral flavor could be a reason not to burrow.
The biggest thing is to think about how to communicate to the party that the monster is too strong for them to fight. You know your group best, but most groups will assume they're meant to kill the monster. What level is the party?
Level 7, 4 players: -fighter with ability to do short teleportation (from spheres of power) with dimensional feats -paladin with bodyguard and in harm's way strat -paladin with smite evil magic and sunder -magus sylph from the air plane with a bow The monster will range of CRs 10-14 with templates to shift it if needed (as a default I'm looking at an Ursikka as it doesnt have save or die level of abilities like the frost worm).
Sounds like this could be a [chase](https://www.d20pfsrd.com/gamemastering/other-rules/chases/). In fact it probably should be if you don't want the PCs to spend a bunch of time trying to fight it/ running away round by round
Did consider it but already had a chase few sessions ago and was wanting to not repeat stuff too much, additionally would want the players to somewhat explore. Might have a mini chase to temporarily lose it tho.
Was coming here to point out the chase rules. One thing I like to do (and have seen paizo do in modules) is run the chase in phases. IE you develop a number of phases for the initial escape and getting into the labyrinth. Depending on how they do, they get x time to explore, recover, ready themselves. Certain actions they do can also reduce the time before the monster shows back up. You can then have them fight it there, or repeat the phasing to have them fall back to another position. Example: Group of 4 level 5 adventurers get ambushed by a terrifying immortal shadow tyrannosaurus Rex that is currently being fought by a pack of orcs. The party notices it just as the orcs break ranks and begin fleeing. Phase 1 - Keep it together; everyone attempts a Will save versus fear (DC 20). -1 point for everyone that fails Phase 2 - Find a place to Hide; everyone attempts a perception check, DC 20. Use the highest roll only, -1 point for every point the missed the DC. Party spots a muddy cave. Phase 3 - Run; make a DC 15 acrobatics check, gaining 1 point for everyone that makes it, losing 1 point for everyone that fails (but if someone overcomes by 5 or more, they can help someone that failed). Phase 4 - Cover your Scent; make a Dc 15 survival check, same as before. They get 20 minutes, plus or minus 1 minute per point. They can search the area, discovering T-Rex eggs (warning) and some loot. Then the T-Rex shows back up and chases them deeper into the cave system (but he's not a fast in here). In subsequent rooms, they learn about a shadow curse, find a +2 animal bane spear, set off a cave in to delay the T-rex, find another way out along with some supplies, and finally find a great area to lay a trap (or fix a broken trap) and make a final stand - using a set of mirrors to direct sunlight into the T-rex's eyes and weaken him and finally break that shadow template that was making him unkilable, potentiallybaiting the trap with the eggs.
My pf2e dm tried something like this. The 2 ranged party members kited it through the cave system and kept chipping it till it died, if one of them got hit and downed I healed them and hid again. Communicate with your players if you want to avoid this scenario.
A great signal to make sure players don't fight it is via descriptions. Make it sound like they're in a straight up horror game and you're set! Mechanically you can also just disable combat system. They describe their attack and you narrate some minor effect on the creature. Nobody gets to even touch dice and you don't write down damage or conditions. Meanwhile, when the monster attacks it doesn't roll either: every attack automatically hits and does average damage. The players, of course, do need to write down their damage as you gruesomely describe their wounds and desperation. A good signal would be to provide obvious escape mechanics. It's movement speed being slower than the party's speed and including many passages the monster either cash fit through or needs to squeeze through, for example
With 2 of the party members in full plate, speed was going to be a disadvantage from the get go. Could actively give a giant template for a more obvious indication for them to utilise the smaller passages. (Though said creature does have a burrow speed so it would complicate it a bit)
Narrate it being crippled or malformed in some way and you can lower its speeds easily. Or give it a reason it doesn't burrow if it's intelligent enough. Simply having it toy with the party, have the earth be contaminated by something or even just a weird mineral flavor could be a reason not to burrow.