Legendary Resistance (4/day). Enter the description for your special trait.

False Appearance. While the onbu remains motionless and laying on the ground, it is indistinguishable from a hill-covered plain or forest.

Charge. If the onbu moves at least 30 feet straight toward a target and then hits it with a horn or headbutt attack on the same turn, the target takes an extra 14 (3d8) damage. If the target is a creature, it must succeed on a DC 17 Strength saving throw or be knocked prone.

Bond of Earth and Hill. The onbu requires half as much food and water as a creature of its size normally would due to it constantly draining small amounts of nutrients from the ground beneath their feet. Additionally, difficult terrain formed by rocky, swampy, or foresty terrain does not cost the onbu extra movement and all of the onbu's ability scores increase by 1 while its AC increases by 2 and it has resistance to acid and force damage while it is in rocky or forest terrain. Also, the onbu has advantage on saving throws against being poisoned, knocked unconscious, and petrified and can walk, run, or jump through nonmagical, unworked earth and stone. While doing so, the onbu doesn't disturb the material it moves through.

Stony Forest Camouflage. The onbu has advantage on Dexterity (Stealth) checks to hide in rocky or foresty terrain. 

Aura of Toxin Neutrility. All toxic plants within 130 ft of the onbu have a 45% chance to lose all of their toxic abilities for 1 year and toxic plants have a hard time growing while the onbu is within 130 ft of them. Also, whenever a poisoned or paralyzed creature that is non-hostile to the onbu comes within 100 ft of it, they are automatically cured of any negative conditions that are affecting them.

Animalistic Communication. The onbu can speak to any unaligned creature of Intelligence 5 or lower as if they had a shared language.

Extreme Challenge. When characters defeat the onbu, they gain 389,000 XP (CR 35) instead of the normal 155,000 XP for defeating a CR 30 creature.

Multiattack. The onbu makes two melee attacks.

Bite. Melee Weapon Attack: +21 to hit, reach 30 ft., 1 target. Hit: 74 (10d12 + 9) piercing damage plus 7 (2d6) radiant or force damage (onbu's choice). 

Side Headbutt. Melee Weapon Attack: +21 to hit, reach 30 ft., 1 target. Hit: 70 (11d10 + 9) bludgeoning damage plus 11 (3d6) piercing damage. 

Horns. Melee Weapon Attack: +21 to hit, reach 30 ft., 1 target. Hit: 70 (11d10 + 9) piercing damage. 

Tail. Melee Weapon Attack: +21 to hit, reach 40 ft., 1 target. Hit: 87 (12d12 + 9) bludgeoning damage and the target must succeed on a DC 21 Strength saving throw or be pushed 10 ft away from the onbu and knocked prone.

Stomp. Melee Weapon Attack: +21 to hit, reach 15 ft., one prone or stunned target. Hit: 68 (9d12 + 9) bludgeoning damage. 

Boulder Spit. RangedWeapon Attack: +23 to hit, range 25/590 ft., 1 target. Hit: 55 (10d10) bludgeoning damage.

Titanic Roar (Recharge 5-6). The onbu lets out a powerful roar that spreads out in a 80-foot cone. Each creature in that radius must succeed on a DC 18 Constitution saving throw, taking 17 (3d10) thunder damage and be frightened and stunned on a failed save, or half as much damage and only be knocked prone instead of stunned and/or frightened on a success.

Super Innate Spellcasting. The onbu's spellcasting ability is Wisdom (spell save DC 19, +11 to hit with spell attacks). The onbu can innately cast the following spells, requiring no components:

At will: speak with plants, aid, animal friendship

3/day: grasping vine

1/day each: commune with nature, greater restoration

The onbu can take 2 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The onbu regains spent legendary actions at the start of its turn.

Detect. The onbu makes a Wisdom (Perception) check.

Tail Smack. The onbu makes a tail attack.

Monstrous Roar (Costs 2 Actions). The onbu uses its Titanic Roar if available.