My party (average level 10) just had a pre-boss fight. That was an opportunity for me to make them spend some of their resources, because they thought that was actually the big bad boss fight.
So we finished the session with them defeating the big bad guy's first mate and they are at the entrance of the cave, inside which resides the boss. The boss will be a wizard about level 14-15. I have heard them planning that they will try to kill him when they meet him, but the actual NPC has heard them for some time now, planning for the event through a successful series of scrying spells. So their plan is to kill him or flee – through teleportation, actually.
My NPC wants them to die, or to surrender to him an artifact they are carrying. So, imagine a wizard of vast intelligence hearing that his enemies have a backup plan of leaving through teleportation. Is there something the NPC could use in his cave – such as spells, magic items or something else – to prevent them from teleporting when things go awry?