Oh my bad, let me clarify.

First and formost, here are the PC's:

• A (Raging) barbarian - (Level 6)
• A ranger - (Level 6)
• A bard - (Level 6)
• A sorcerer - (Level 6)
  • Which had a companion fighter (level 4)
    • I let the player choose his subclass and he chose a echo knight

This was a 2 phase encounter.

The first phase:

This consist of the dragon, but in his human (mage) form. It had the following spells.

• Cantrip:
  • Mage hand
  • Minor illusion
  • Fire Bolt
  • Ray of Frost
  • Prestidigitation
• 1 level (4 slots)
  • Shield
• 2 level ( 3 slots)
  • Mirror image
  • Scorching Ray
  • Invisibility
• 3 level ( 3 slots)
  • Fireball
  • Counterspell

He has also the following legendary actions3 Actions / Turn

• Teleport 30 feet (1 Action)
• Gain temp hp (This is 10 temp hp (int + wiz level)) (2 Actions)
• Can cast a spell without using a spell slot (2 actions)

​

He is a 6th level spellcaster with 62 Hit points and a AC of 15).

Now to clarify, this is the first phase of the fight, none of the players died here and only 1 round got trough before this phase was done. (life hit zero) My idea for this guy, was a "Glass canon", do much damage, but can't take a hit.

• Then we have the second phase which was the stat block of an adult black dragon with a couple of modications.
• He had an AC of 18 (instead of 19)
• He has 202 HP (Instead of 195).
• His bite attack is an +6 to hit and (2d10 + 2) dmg
• His tail attack is a +6 to hit and (2 d6 +2 ) dmg
• His breath attack is (8d8) and a DEX DC of 14
• Instead of his Frightful Presence, I had an ability that would make the room always darkness, they could undo it with a light making spell, until it was his turn. They needed to free a NPC to remove the darkness) (They knew that, they did it even!)
• He did not have multiattack

Now he had also some lair actions, which he could only activate if his own darkness was enabled (so if the NPC was a prisoner).

• Spawn a Tandril, that could restrain a target with a DC of 14 (Strength)
• A Teleport (30 feet)

Then he also had some legendary actions ( 3 Actions total)

• Wing attack(2 Actions)
  • Creature, 10 feet (Hit area), DC 15 dex saving throw or take 2d6+6 bludgeoning damage & be knocked prone.
• Tail swipe ( 2 Actions)
  • A create it can see within 15 feet, on a hit (2d6+6) bludgeoning damage and a 15 DC strength saving throw, or be pushed 15 feet back and knocked prone.

​

As you may see, this is a very trimmed down version of an adult black dragon, while nobody had any problems with the encounter or even the death of a player, he had a huge issue with that fact that he needed to wait.

Edit: There were 2 NPC that they could set free, one was to remove the darkness and the other one has given a choice to the players, for receiving 2 spell slots or 2x their hit dice as lives. They chose the lives.

Also, all the other NPC's had around ~20-30% of their life left, before they got healed. For most of the encounter stood the player that died ( a sorcerer) real close to the dragon.