I am currently DMing a D&D 5e campaign for a party of 4, and I wanted a villain with a teleporting ability similar to Nightcrawler, from X-Men/Marvel Comics.
I have run into an issue. The main attraction for me of a teleporting character is that they would be difficult to hit, due to the ability to teleport out of the way of an attack, but I am having trouble balancing a reaction-based teleporting ability - think of the Parry maneuver, where you can take a reaction to up your AC, but here instead you teleport out of harm's way.
The problem with balancing this is that if I make it powerful enough to be effective, then it becomes nigh-impossible to hit said villain. A solution would be to limit the amount of times he can do it, but that decreases the appeal of an NPC built upon his unlimited uses of teleportation powers.
How can I give a villainous NPC this sort of ability without it being overpowered?