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.
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.