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.
Medium beast, same alignment as the necromancer who brought them back to life
AC: 10 (unarmored)
Speed: 65 feet
STR DEX CON INT WIS CHA 15 (+2) 24 (+7) 10 (+0) 30 (+10) 30 (+10) 6 (-2)
Senses: Truesight 200 feet
Challenge 15 (13,000 XP)
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
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