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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?

Ok, there's two ways to go about it. Limited use, or limited reliability.

Limited Use

This is simple: as a reaction, teleport up to your move. Reaction rules limits this to 1/round.

Limited Reliability

Follow the 3.5's Wall of Blades 'spell' example: opposed rolls. Pick a save that makes sense (might be Dexterity, for reaction, or Intelligence, for the focus required, for example). I'd probably go with Dex. As a free action, when attacked, roll a Dex save. If this is more than the incoming attack's roll, teleport 5ft vs ranged or your move speed vs melee, avoiding the attack (ranged vs melee difference to keep the enemy from moving away too fast when the melee haven't even caught up yet). Limit this to when you're aware of the attacker and able to react (or in other words, when the enemy doesn't have advantage on their attack).

In any case, go with a pitifully low AC, especially with the second effect. It amounts to a 50% miss chance, pretty much enough to keep things ok

The Prestige Way

This is actually like a magic trick, in that under the hood, no teleportation-dodge happens. It just looks that way.

Build the villain for a high AC through Dexterity. Give him 'when an attack misses you or you pass a dexterity save, you can move X feet'. Describe misses as missing (and saves passed) because of the teleportation, not the other way around. Roll a die behind your screen when the players attack, and pretend to care of the result and blame it for the 'active' misses.

Give your villain the blink spell

The blink spell description says:

Roll a d20 at the end of each of your turns for the duration of the spell. On a roll of 11 or higher, you vanish from your current plane of existence and appear in the Ethereal Plane (the spell fails and the casting is wasted if you were already on that plane). At the start of your next turn, and when the spell ends if you are on the Ethereal Plane, you return to an unoccupied space of your choice that you can see within 10 feet of the space you vanished from.

Though it isn't teleporting per se, you can reflavor it as such. You can make it at-will spell or part of innate spellcasting with limited number of uses, though spell balances itself just right, as the probability of spell's effect occurring is 50%.

Throw in a couple of Rogue's class features

Uncanny Dodge

Starting at 5th level, when an attacker that you can see hits you with an attack, you can use your reaction to halve the attack's damage against you.

Evasion

Beginning at 7th level, you can nimbly dodge out of the way of certain area effects, such as red dragon's fiery breath or an ice storm spell. When you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you instead take no damage if you succeed on the saving throw, and only half damage if you fail.

Treat these as constant blinking, making it hard for his opponents to land a solid hit.

Overall your character seems like a reflavored 13th level Arcane Trickster, except for his dodging is actually teleporting away in the nick of time.

I haven't tried this or seen it in action myself, but the Mystic class from Unearthed Arcana has a lot of superhero-style abilities, classified in-game as psionics. In particular, the Nomadic Step psionic discipline has a lot of teleportation abilities, including the Defensive Step psionic effect, which seems quite similar to what you want:

Defensive Step (2 psi). When you are hit by an attack, you can use
your reaction to gain a +4 bonus to AC against that attack, possibly
turning it into a miss. You then teleport up to 10 feet to an
unoccupied space you can see.

I know a lot of DMs avoid UA for player characters due to possible issues with balance, but for an NPC you control you might consider it a bit more kosher, certainly moreso than a complete homebrew.

Make a it a legendary ability.

I like to give all boss creatures legendary actions, makes them feel more epic compared to being just a stronger mob.

this is the table i use

• move action - 1 point


• single attack action 1 point


• cast a cantrip - 1 point


• dash action - 2 points


• multiattack - 2 points


• cast a spell - 2 points


• move and attack action - 3 points

chose how may points you boss has (1-4) based on difficult tier.

then just make you teleport a cantrip
- The next time the creature is hit with an attack the attack misses and the creature teleport's to an open space it can see up to 30 ft away.

As it is a legendary action it can only be taken at the end of a players turn so you will have to preempt when you want to cast it so it wont always be active.

it only affects the next hit so you can only dodge once per player turn making it possible to hit the boss while keeping the nightcrawler theme.

PS this is a great idea for a boss and im looking forward to running it soon :)