Boss Encounter 10 - lvl 5 to 10

Erinyes Medium fiend (devil) , lawful evil
Armor Class 18 (Plate) Hit Points (18d8 + 72) Speed 30 ft., fly 60 ft.
STR 18 (+4) DEX 16 (+3) CON 18 (+4) INT 14 (+2) WIS 14 (+2) CHA 18 (+4)
Saving Throws DEX +7, CON +8, WIS +6, CHA +8 Damage Resistances Fire, Cold; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks that aren't Silvered Damage Immunities Poison Condition Immunities Poisoned Senses Truesight 120 ft., Passive Perception 12 Languages Infernal, Telepathy 120 ft. Challenge 12 (8,400 XP) Proficiency Bonus +4
Hellish Weapons. The erinyes's weapon attacks are magical and deal an extra (3d6) poison damage on a hit (included in the attacks).
Magic Resistance. The erinyes has advantage on saving throws against spells and other magical effects.
Actions Multiattack. The erinyes makes two attacks.
Longsword. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: (1d8 + 4) slashing damage, or (1d10 + 4) slashing damage if used with two hands, plus (3d6) poison damage.
Longbow. Ranged Weapon Attack: +7 to hit, range 150/600 ft., one target. Hit: (1d8 + 3) piercing damage plus (3d6) poison damage, and the target must succeed on a DC 14 Constitution saving throw or be poisoned. The poison lasts until it is removed by the lesser restoration spell or similar magic.
Reactions Parry. The erinyes adds 4 to its AC against one melee attack that would hit it. To do so, the erinyes must see the attacker and be wielding a melee weapon.