Boss Encounter 11 - lvl 5 to 10

1x Behemoth Huge beast , neutral evil
Armor Class 15 (Natural Armor) Hit Points (10d12 + 200) Speed 50 ft., climb 20 ft., swim 25 ft.
STR 25 (+7) DEX 12 (+1) CON 20 (+5) INT 8 (-1) WIS 14 (+2) CHA 9 (-1)
Saving Throws STR +11, CON +9, WIS +6 Skills Athletics +10, Perception +4 Damage Resistances Bludgeoning, Cold, Ranged Attacks Condition Immunities Frightened, Paralyzed, Petrified Senses Darkvision 35 ft., Passive Perception 14 Languages -- Challenge 12 (8,400 XP) Proficiency Bonus +4
Siege Monster. The Behemoth deals double damage to objects and structures.
Magic Resistance. The Behemoth has advantage on saving throws against spells and other magical effects.
Momentum. Swimming speed increases by 5 feet every turn spent moving in water, up to 50 ft. When transitioning between land and water, the Behemoth may carry its speed unless it changes directions.
Terrifying Call. Both Frightful Bark and Aggressive Roar cause all non-intelligent sentient mounts to flee in fear. Flying mounts may be guided towards the ground for riders to disembark, but must continue to move away from the Behemoth during their descent.
Actions Multiattack. The Behemoth can make two attacks per turn, excluding breakthrough and gore.
Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: (4d8 + 7) piercing damage. Target must make a DC 14 CON saving throw or be poisoned for 3 turns. If the target is a Medium or smaller creature, it is grappled (escape DC 15 DEX). Until this grapple ends, the target is restrained, and the Behemoth can't bite another target.
Thrash. Melee Weapon Attack: +12 to hit, reach 5 ft., strike all targets in range. Hit: (4d6 + 5) bludgeoning damage. All targets in range must make a DC 15 DEX saving throw or be blinded for 2 turns. If a target is grappled, the target takes an additional (4d6 + 7) piercing damage and must make another DC 14 CON saving throw or be poisoned for 3 turns.
Claw. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: (4d6 + 7) slashing damage. On a successful hit, target must make a DC 14 CON saving roll or be poisoned for 3 turns.
Tail. Melee Weapon Attack: +12 to hit, sweep a 15 foot cone. Hit: (3d8 + 7) bludgeoning damage. All targets must pass a DC 15 DEX saving roll or be knocked prone and be knocked back up to 10 feet.
Breakthrough. Melee Weapon Attack: +12 to hit. Must be engaged in melee, have not acted this turn, or moved for two turns. Roll a DC 15 DEX saving throw. On success, choose a point within 40 feet and move there. Do (3d8 + 7) bludgeoning damage to all targets on a 15-foot wide line between the Behemoth's starting and finishing positions. Targets must also roll a DC 15 DEX saving throw or be knocked prone. On a fail, end turn immediately and do not move.
Gore. Melee Weapon Attack: Target up to two targets adjacent to each other. If the Behemoth has not moved this turn, it may charge its target(s), upon failed DC 15 DEX save target(s) take 1d8 piercing damage for every 5 feet travelled.
Frightful Bark. Every creature within 80 feet must make a DC 14 WIS saving throw or become frightened for 1 minute. A creature can make a DC 14 WIS saving throw at the end of each of its turns, ending the effect on itself on a success. Creatures must also make a DC 16 CON saving throw or become deafened for 1 minute and take 1d4 force damage. If a creature makes a successful Wisdom saving throw, they become immune to the frightened effect from Frightful Bark for 24 hours, but may still be affected by the deafening effect.
Aggressive Roar (Recharge 4-6). The behemoth lets out a roar of horrifying ferocity in a 80-foot cone. Each creature in that area must make a DC 18 Constitution saving throw, taking (8d8 + 7) force damage and becoming deafened for 1 minute on a failed save, or half as much damage on a successful one. Every creature must also make a DC 18 Wisdom saving throw or become frightened for 1 minute. This effect may be ended early following the same rules as in Frightful Bark. This ability cannot be used while engaged in melee, and the Behemoth is extremely unlikely to use it when an enemy is within charging distance.