Is this homebrew “tankkiller” balanced? [closed]

Question

Normally my party has a couple players tanking, and others staying behind and hiding, and from what I can tell this is relatively common. To mix things up every now and then, I have come up with the following homebrew:

The Tankkiller was a battlemage in their previous life, specifically one who specialized in dark magic. They would sap the strength of their opponents and use it against them, and were nearly impossible to kill. A Tankkiller is one of these battlemages who has been brought back from the dead by a master necromancer. Nobody knows what these Tankkillers look like except that they are humanoid, as they always hide behind a dark, full body cloak, and use their dark powers to disappear whenever they are in danger.

Tankkiller

Medium beast, same alignment as the necromancer who brought them back to life

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AC: 10 (unarmored)

HP: 0

Speed: 65 feet

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STR	DEX	CON	INT	WIS	CHA
15 (+2)	24 (+7)	10 (+0)	30 (+10)	30 (+10)	6 (-2)

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Senses: Truesight 200 feet

Languages: none

Challenge 15 (13,000 XP)

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Incorporeal: When at 0 hit points and not recovering from combat, the Tankkiller cannot be damaged or killed

Teleport: When not recovering from combat, the Tankkiller can expend all of its hit points (even if it has 0) to teleport anywhere within 200 miles. This returns it to 0 hitpoints and makes it start recovering from combat for two hours

Creature of doom: does not have to make death saving throws at 0 hitpoints, instantly dies when it goes down to -1 hitpoints

Actions

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Punch. Melee Weapon Attack: guaranteed, reach 5 ft., one target. Hit: 10% of the targets hitpoints. Gives hitpoints equal to 1/3 the damage dealt (rounded up). If this is the Tankkillers first time attacking a particular opponent an extra 15 hitpoints are gained.

Shadow bringer Spell attack: guaranteed hit, hits all enemies within 100 feet of the Tankkiller. Reduces the constitution of all enemies within 100 feet of the Tankkiller by 1 for 5 turns, can stack up to 10 times on a target. Using this attack deals 200 damage to the Tankkiller

Note: my biggest concern (other than combat of course) is that the teleport ability might be too powerful

Answer 1

As written, this creature is entirely indestructible. It starts at 0 hit points, and you've written that when at 0 hit points it "cannot be damaged or killed".

You've written a "Shadow Bringer" attack which isn't very dangerous. Even if you had two of these creatures doing it for five rounds, inflicting the full -10 CON, that wouldn't actually kill anyone. It also deals a bunch of damage to the creature that does it, which is irrelevant because the creature ignores all damage. There doesn't seem to be much point to this.

"Guaranteed hit" attacks are bad and boring. They tell your players: "You might as well have not bothered investing in armor because the creatures are going to completely ignore it."

It sounds like, if an adventuring party ran into one of these, they'd be completely unable to harm it and it would gradually punch them all to death unless they had a way to teleport away from it. That doesn't sound like it would be fun at all.

There are rules for determining a creature's CR: you compute its "offensive CR" based on how much damage it does, and its "defensive CR" based on how much damage it can take before it dies. You should read those rules before doing any more homebrew monsters.

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If your goal is to "mix things up a little" regarding the group's use of tanks, you should build a combat where creatures attack from all sides and there's no way to hide "behind" the tank. Note that this can easily get your group killed -- there's a reason they hide behind the tank most of the time -- so you'll want to be careful that the resulting battle isn't too deadly.