Feral Instinct: Zamtarak, Elemental Terror of Earth has advantage on initiative rolls. Also, if he is surprised at the beginning of combat but not incapacitated, he can act normally on his first turn if he enters a rage before doing anything else.

Legendary Resistance (3/Day). If Zamtarak, Elemental Terror of Earth fails a saving throw, it can choose to succeed instead.

Spellcasting. Zamtarak, Elemental Terror of Earth is a 10th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 20, +14 to hit with spell attacks). It has the following druid spells prepared:

Cantrips (at will): Druidcraft, Magic Stone, Mold Earth, Shillelagh

1st level (4 slots): Cure Wounds, Earth Tremor, Entangle, Longstrider 

2nd level (3 slots): Earthbind, Heat Metal, Dust Devil, 

3rd Level (3 slots): Meld Into Stone, Plant Growth, Erupting Earth

4th Level (3 slots): Guardian of Nature, Stone Shape, Stone Skin
5th Level (2 slots): Transmute Rock, Wall of Stone

Multiattack: Zamtarak, Elemental Terror of Earth can make three attacks with his maul or two attacks with Magic Stone. 

Maul +3: Melee Weapon Attack:+19 to hit, reach 5 ft., one target. Hit:  22 (3d6+12) bludgeoning damage. Zamtarak, Elemental Terror of Earth can roll one additional weapon damage die when determining the extra damage for a critical hit with this weapon. 

Magic Stone: Melee Weapon Attack: +17 to hit, range 60 ft., one target. Hit:  14 (1d6+10) bludgeoning damage.

Reckless. At the start of its turn, Zamtarak, Elemental Terror of Earth can gain advantage on all melee weapon attack rolls during that turn, but attack rolls against him have advantage until the start of its next turn.

Zamtarak's Rage (Recharges after a Short or Long Rest). As a bonus action, Zamtarak, Elemental Terror of Earth can enter a rage at the start of its turn. The rage lasts for 1 minute or until Zamtarak, Elemental Terror of Earth is incapacitated. While raging, Zamtarak, Elemental Terror of Earth gains the following benefits:

• Zamtarak, Elemental Terror of Earth has advantage on Strength checks and Strength saving throws
• When he makes a melee weapon attack, Zamtarak, Elemental Terror of Earth gains a +4 bonus to the damage roll.
• Zamtarak, Elemental Terror of Earth has resistance to all damage types except psychic. 

Wild Shape (Recharges after a Short or Long rest). As a bonus action, Zamtarak, Elemental Terror of Earth can assume the shape of any beast (CR 3 or Lower - EX Polar Bear, Rhinoceros, Giant Scorpion) or an Earth Elemental. he can stay in this form for 3 hours or until he reverts to his normal form as a bonus action. He automatically reverts if he falls unconscious, drops to 0 hit points, or dies.

While transformed, Zamtarak, Elemental Terror of Earth game statistics are replaced by the statistics of the beast or elemental, except he retains his alignment, personality, and Intelligence, Wisdom, and Charisma scores. His attacks in beast form are magical. While in beast form, Zamtarak, Elemental Terror of Earth can use a bonus action to expend one spell slot and regain 1d8 hit points per level of the spell slot expended.

Stone of Controlling Earth Elementals: If the stone is touching the ground, you can use an action to speak its command word and summon an earth elemental, as if you had cast the conjure elemental spell. 

Staff of the Woodlands:  This staff has 10 charges for the following properties. It regains 1d6 + 4 expended charges daily at dawn. If you expend the last charge, roll a d20. On a 1, the staff loses its properties and becomes a nonmagical quarterstaff.

Spells. You can use an action to expend 1 or more of the staff's charges to cast one of the following spells from it, using your spell save DC: animal friendship (1 charge), awaken (5 charges), barkskin (2 charges), locate animals or plants (2 charges), speak with animals (1 charge), speak with plants (3 charges), or wall of thorns (6 charges).

You can also use an action to cast the pass without trace spell from the staff without using any charges.

Tree Form. You can use an action to plant one end of the staff in fertile earth and expend 1 charge to transform the staff into a healthy tree. The tree is 60 feet tall and has a 5-foot-diameter trunk, and its branches at the top spread out in a 20-foot radius. The tree appears ordinary but radiates a faint aura of transmutation magic if targeted by detect magic. While touching the tree and using another action to speak its command word, you return the staff to its normal form. Any creature in the tree falls when it reverts to a staff.

Zamtarak, Elemental Terror of Earth can take 3 legendary actions. Only one legendary action option can be used at a time and only at the end of another creature's turn. Zamtarak, Elemental Terror of Earth regains spent legendary actions at the start of his turn.

Magic Stone: Zamtarak, Elemental Terror of Earth can throw a Magic Stone. 

Strike: Zamtarak, Elemental Terror of Earth can make one weapon attack with his Maul. 

Move: Zamtarak, Elemental Terror of Earth moves up to his movement speed without provoking opportunity attacks.