How can I design a boss that shifts between two dimensions to avoid attacks?

Question

This boss character, Omega, is inspired by

• Zhylaw, leader of the Necromongers, in the Riddick film series
• Tobi, Akatsuki member from the Naruto anime

This boss, meant for a level 16 boss fight (where players enter tier 4 play), has the ability to shift between the Material Plane and the Shadowfell at will. The adventure has the group separating, 4~5 fight in the Shadowfell, and 4~5 in the Material Plane.

Narratively, it's incredible to describe how Omega fights.

You throw a spear at him, and immediately see that his body glows and shimmers briefly, and your attack simples passes through him, unphased. Is it an illusion, you wonder? Yet, he smiles, jumps in the air, and throws a projectile in your direction. The very sound of the wind whistling as the projectile flies through the air makes it obvious the projectile is not an illusion, and you'd better dodge it somehow!

Basically, just like Tobi, moves his body onto the Shadowfell when being attacked. He avoids the attack, and then reappears in the Material Plane to go on the offensive. Here's an example:

https://youtu.be/H3SIGhEyQqo?t=407

The end of the fight, when the PCs finally defeat him, should also be a fantastic climax, where Omega is getting attacked in both dimensions at the same time, and needs to choose in which it will die.

https://www.youtube.com/watch?v=yX5TsLuIEy8

I have tried this encounter once, but it fell flat. I basically had my players on the Shadowfell, and their archnemesis group in the Material Plane (both groups had to unite to defeat him). I gave Omega the following ability:

At the beginning or end of every turn, Omega can planeshift, dealing 2d10 force damage on the locations he leaves and enters at, and pushing enemies away from where he appears.

This basically created a slow slog fest, where everyone was just readying their attacks and attacking every other turn. Worst of all, it didn't quite feel like the strategic shift to avoid an attack.

I think I want to move this ability into a Reaction (instead of it being at beginning or start of turns), and also give Omega the ability to take as many Reactions per round as he wants to (although at most one Reaction per turn). That way, he could plane-shift when he was about to take damage.

I wonder if there is a better way to frame his powers within the ruleset of 5e.

• An ability that already exists in a published monster, and which is not relying on infinite reactions (which I think is not something any monster has?). Or maybe reflavoring an existing ability to match the description of his powers.
• Or homebrewed abilities that other DMs have tried and tested. What did the experience go? Any pros and cons of that session? Did the players enjoy it, and could it be used to emulate Omega's powers?

Answer 1

In a word, don't

The closest I could find to creatures that can move in and out are things like ghost and succubus:

Etherealness. The ghost enters the Ethereal Plane from the Material Plane, or vice versa. It is visible on the Material Plane while it is in the Border Ethereal, and vice versa, yet it can't affect or be affected by anything on the other plane.

Etherealness. The fiend magically enters the Ethereal Plane from the Material Plane, or vice versa.

But both of these have the phasing happening as an Action; not at-will.

Part of the problem with what you're describing is that, despite being described as happening simultaneously, combat in DnD5e is handled turn-based. Creature-A goes, then Creature-B, then Creature-C. So if Omega can phase in and out at-will, there is no way an individual player can hit them. The proof is in your narrative example; player attacks and misses, but Omega can immediately phase back in to retaliate.

The next issue is you want to have competing parties on both planes coordinate their attacks. Which as you pointed out...

1. They are competing parties so less likely to help and coordinate with each other.
2. Half the players need to use the Ready Action meaning they lose out on multiple attacks and cannot change tactics as the fight progress.

As Darth Pseudonym pointed out in the comments, communication between parties will be a huge problem. If Omega was switching back and forth between the Ethereal plane, there are methods to see both sides at once so the parties could use sign language to plan attacks. I am not aware of any spell or feature that would work with the Shadow plane.

This method also means you take away some player agency. Doolgar and Lothar both attack Omega at the same time. You get to decide in the moment which damage Omega should take so attacking is no longer strategy, but a crap-shoot. Why should Lothar bother using his big spells when Omega will just phase to the other side and take Doolgar's sword swing instead?

If you still want to go down this route, use Legendary Actions instead

Legendary Actions are a built-in mechanic so you're not trying to invent something new. It's a way to even the odds when the action economy is way out of whack (1 BBEG vs 8-10 creatures--some with multiple attacks--focused solely on one creature).

Give Omega 4-5 Legendary Actions per round. Each allowing Omega to phase to the other side. This would average out to crossing over every two player attacks.

The trick is to keep the players guessing about what side Omega is really on. So at the end of every player's turn Omega will "shimmer" (or whatever you want to call it). Sometimes he uses the LA to cross over, sometimes he does not.

This moves the combat from crap-shoot to a shell game. Now there can be strategy. Now players can pick and choose if they want to coordinate. It's no longer up to the DM to decide what damage is taken.

And while I haven't done this exact scenario, I have run something similar. My suggestion is that you keep a coin handy; heads Omega is in the Prime plane, tails Omega is in the Shadow plane.

Answer 2

What you described sounds very much like a Blink spell, but 3.5 edition of it

Physical attacks against you have a 50% miss chance, and the Blind-Fight feat doesn’t help opponents, since you’re ethereal and not merely invisible. If the attack is capable of striking ethereal creatures, the miss chance is only 20% (for concealment).

This miss chance was exactly from a character being on another plane of existence. In my experience, it worked reasonably well. As with any other defence mechanism, fighting was longer, but there was no need to mess with initiative, prepare attacks, et cetera.

Miss chance was generally converted to a disadvantage between 3.5 and 5e, and I believe, but not playtested, that would work well.

I tried to play with more than one reaction, but to be honest it only complicated the action economy back to 3.5 level. I don't believe it is the way to go. And more importantly, that's not what D&D designers decided it was a way to go, apparently. I strongly recommend against such regressions.

Answer 3

If you want to take a slightly different approach to this problem, I think you could allow him to shift, but it functions exactly like Uncanny Dodge as a reaction and let him take these as often as he likes. This puts his abilities more inline with existing 5e mechanics.

Uncanny Dodge requires the Attacker see the incoming damage, which can give your players fun methods of trying to strike unseen: invisibility, sneaking, or even ruling he can't look both ways while flanked.