In one of the later one-on-one sessions, my dad's characters (6th level, operating as the leaders of their village) made an alliance with one of the great merchant houses.
The two groups would work together to collect volcanic ash and soil from near an active volcano, then split the cargo between their two groups.
The villager would use the stuff to fertilize their farmlands, and the merchant house would sell their portion in the big city to the Mage-King for a MASSIVE profit.
The problem: the volcano was home to an Efreeti Noble (referred to in-campaign as the "spirit king" of the volcano) known for refusing to allow people to collect "the riches of his kingdom" unless they first proved themselves in combat.
There were three combat encounters, and the last one was against this guy:
DRAGON, CHAMPION OF AGNI
Large dragon
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Armor Class: 16 (natural armor)
Hit Points: 172 (15d10 + 90)
Speed: 60 ft., climb 40 ft., fly 60 ft.
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STR 22 (+6) DEX 14 (+2)
CON 22 (+6) INT 12 (+1)
WIS 18 (+4) CHA 18 (+4)
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Saving Throws Constitution +19, Charisma +8
Skills Athletics +10, Insight +8, Perception +12
Damage Immunities acid, fire
Conditional Immunities frightened, paralyzed, poisoned
Senses Blindsight 60 ft., Truesight 15 ft., passive Perception 23
Languages Common, Draconic
Challenge 10 (5,900 XP)
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Acidic Blood. Whenever the dragon takes damage of any type except psychic, each creature within 5 feet of it takes 7 (2d6) acid damage.
Magic Weapons. All of the dragon's weapon attacks are magical.
Innate Spellcasting. The dragon's innate spellcasting ability is Charisma (spell save DC 16). It can innately cast the following spells, requiring no material components:
At will: detect magic, identify, major image
1/day each: dispel magic, fear, locate object
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ACTIONS
Multiattack. The dragon makes three attacks; two with its claws, and one with its bite.
Bite. Melee Weapon Attack: +10 to hit, reach 10 ft.
Hit: 15 (2d8 + 6) piercing damage plus 9 (2d8) acid damage, and if the target is of Large size or smaller, it is grappled and restrained (escape DC 18). While the target is grappled in this way, the dragon can automatically deal 15 (2d8 + 6) piercing damage plus 9 (2d8) acid damage to the target on each of its subsequent turns, and it cannot use its bite against another creature.
Claw. Melee Weapon Attack: +10 to hit, reach 10 ft.
Hit: 12 (1d12 + 6) slashing damage, and the target must make a DC 18 Strength saving throw or be pushed 5 feet away and knocked prone.
Tail. Melee Weapon Attack: +10 to hit, reach 10 ft.
Hit: 10 (1d8 + 6) bludgeoning damage.
Breath Weapon (1d6 recharge, 5--6). The dragon exhales a 60-foot cone of destructive energy. Each creature within the area takes 22 (4d10) fire damage, and must make a DC 18 Dexterity saving throw, taking 15 (6d4) acid damage on a failure, or half as much on a success (37 failure, 30 success).
At the start of their next turn, each creature that was within the area must make a DC 18 Constitution saving throw or be poisoned until the start of their following turn.
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LEGENDARY ACTIONS
The dragon has 3 legendary actions (1/round each), which it can only use at the end of another creature's turn.
Spell. The dragon casts a spell.
Wings.. The dragon beats its wings. Each creature within 10 feet of the dragon must make a DC 18 Strength saving throw or be pushed 10 feet away. The dragon can then fly up to half its flying speed.
Tail Sweep.. The dragon makes one tail attack.
By the end of the conflict, the Druid's face was burned to a crisp and he was on the floor making death saving throws, and the other three (evocation wizard, arcane trickster rogue, storm herald [fire] barbarian) were all down to 10 hit points or lower.