Boss Encounter 6 - lvl 5 to 10

Horned Devil Large fiend (devil) , lawful evil
Armor Class 18 (Natural Armor) Hit Points (17d10 + 85) Speed 20 ft., fly 60 ft.
STR 22 (+6) DEX 17 (+3) CON 21 (+5) INT 12 (+1) WIS 16 (+3) CHA 17 (+3)
Saving Throws STR +10, DEX +7, WIS +7, CHA +7 Damage Resistances Fire, Cold; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks that aren't Silvered Damage Immunities Poison Condition Immunities Poisoned Senses Darkvision 120 ft., Passive Perception 13 Languages Infernal, Telepathy 120 ft. Challenge 11 (7,200 XP) Proficiency Bonus +4
Devil's Sight. Magical darkness doesn't impede the devil's darkvision.
Magic Resistance. The devil has advantage on saving throws against spells and other magical effects.
Actions
Fork. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: (2d8 + 6) piercing damage.
Tail. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: (1d8 + 6) piercing damage. If the target is a creature other than an undead or a construct, it must succeed on a DC 14 Constitution saving throw or lose (3d6) hit points at the start of each of its turns due to an infernal wound. Each time the devil hits the wounded target with this attack, the damage dealt by the wound increases by (3d6). Any creature can take an action to stanch the wound with a successful DC 12 Wisdom (Medicine) check. The wound also closes if the target receives magical healing.
Hurl Flame. Ranged Spell Attack: +7 to hit, range 150 ft., one target. Hit: (4d6) fire damage. If the target is a flammable object that isn't being worn or carried, it also catches fire.