How to make a "Catch the monster" tabletop encounter properly challenging?

Question

How do I make a DnD 5th edition "Catch the monster" tabletop encounter properly challenging for the PCs? Specifically, what are good ways to thematically prevent a pair of level 6, 5th edition rogues from winning a game of "tag" with a monster? I'm envisioning having other monsters provide cover for the fleeing creature, what else could be done?

I'm prepping an encounter where the combat is unwinnable and the clearly-indicated only way out is for the PCs to catch a monster that's holding a portkey-style artifact.

Setting

The group of 4 PCs have been lead into an ambush in a ruined courtyard. Their Mindflayer nemesis has prepared a coterie of henchmen to finish them off. The Mindflayer knows they have a teleportation artifact and so his surprise-round move is to have his hidden Hook Horror pounce on the PC with the portkey, rip his backpack off, and start running it across the lengthy ruined courtyard to the Mindflayer. Completely unhindered I was thinking of the Hook Horror's route taking it 3 turns to reach the Mindflayer.
The PCs will have the Mindflayer's henchmen raining darts and eyebeams down on them from fortified positions on either side.

How do I make this encounter enjoyable, without the PCs feeling its impossible or (bigger concern) immediately catching the Hook Horror and fleeing in 1 turn?

The party is lvl 6. The comp is 2 rogues (halfling and tabaxi), 1 dwarf paladin, and 1 elf warlock.

Answer 1

Based on how you worded this scenario, you're looking to build a tightly-timed race with a monster as the only way to successfully disengage from combat, with the PCs having only three rounds to catch the necessary object while in motion. Three rounds isn't a lot of time, and as Trish noted in the comments, this can go a bunch of ways depending on party spec.

Things to remember about enemies: if the PCs can think of it, so can they. Impeding motion by blockades or caltrops? Stationing allies along the path to assist the HH? Focusing fire on the party member obviously going after the necessary item? The Mindflayer, at least, is smart enough to think of the same basic tactics the party would under similar circumstances, and can direct its henchmen accordingly.

Other tactical things to consider:

• Once they catch up to the HH, they still have to get the backpack back from it somehow

• Once they have it back, they still have to regroup with the rest of the party, fish the portkey item out of the bag, and actually use it, all while taking fire from everybody and probably being chased back by the HH.

There's still plenty of potential for things to get hairy in the time between regaining the object and actually using it.

While you're thinking of ways to impede the PCs, make sure you think of ways to not trap them in a TPK if things go wrong (assuming, of course, this is not the kind of game where that's OK). Make sure your worry about ending the encounter too soon doesn't eclipse your desire to have your players enjoy it.

All this being said, KorvinStarmast has a point that a discussion-based forum is likely going to be more helpful than here, as you likely need more conversation and brainstorming than this format is meant for.

Answer 2

There are a lot of potential problems here:

• How does the mind flayer know which backpack has the portkey? What if the mind flayer guesses wrong?
• How hard is it to rip someone's backpack off? There are no rules for this, so you're going to have to houserule something. Are you going to give the character a save to resist getting their stuff stolen? If not, the players might feel like you're cheating against them.
• The rogues are faster than the hook horror, and they ought to be good at taking stuff from people; can't they just dash up and take the backpack back?
• What if the group gets separated during the battle? We don't know how your artifact works, but is there a chance they might teleport out with part of the group, leaving someone to die?
• D&D combats can end pretty fast. It sounds like you want to have a group of monsters that won't immediately kill the group, but also are clearly far more powerful than them. Are you sure this works?
• It sounds like you're planning a scenario where the group will be badly outmatched and will struggle to even escape with their lives. Is this really going to be fun for them? They might not like this.

I'd like to propose an alternate solution to your problem. I think your mind flayer doesn't know who has the portkey, and he doesn't want to rely on trying to rip people's backpacks off. Instead, he's got some sort of way to prevent teleportation.

Note that D&D has lots of anti-teleportation effects in it already. One example is the private sanctum spell, which shuts down teleportation in a 100-foot cube at the cost of a fourth-level spell slot. You probably don't want to use these effects directly because they're annoyingly binary -- either the wizard casts a successful dispel magic, or else everyone dies. Instead you should make some sort of challenge involving several glowy magic things which need to be killed, broken, or disabled.

Perhaps there's a great big magic circle surrounding the courtyard, with six runes in prominent positions which are providing power to the anti-teleportation effect. Or there are some glowy magic pillars which have private sanctum or hallow tied to them. Or there are adepts looking down on the courtyard from above, and their eyes cast a pale blue light which prevents teleportation.

So the group walks into the courtyard and immediately see the glowy magic thing. You can let them roll skill checks to figure out that the thing is a teleportation inhibitor. Then the adventure becomes a challenge where they have to wreck all N of the teleportation inhibitors so they can teleport out.

Good luck with it.

Answer 3

a problem I see with this are the differences in speed - especially the Tabaxi rogue: with a base speed of 30, and the cunning action, both of the rogues have the ability to move faster than the hook horror, which does not have cunning action. Furthermore, the Tabaxi's racial trait allows him/her to double their speed temporarily, practically allowing him/her to catch up immediately. My suggestion would be to integrate the hook horror's climbing speed (30) as opposed to the tabaxi's (20), perhaps in conjunction with the mind flayer's (at will) levitate - which could assist the hook horror. Since the encounter occurs in a ruined courtyard, you could also potentially have the ground be unstable, so running recklessly after the hook horror could cause the floor to cave in, leading to a separate area of the map. You could even have areas of the fortified courtyard be of wood, so the warlock could hinder the hook horror by burning places down.