Tower Shield. The Warden can use a reaction to grant itself full cover from any frontal attacks.

This shield also grants Advantage on Saves against Cone attacks.

Adamantine Resistance  The warden is immune to critical hits and is immune to all attacks from non magical weapons. The wardens attack as a result of its construction bypass Adamantine armour resistances.

Stalwart The warden due to its immense size and weight is unable to be moved by any physical means  

Multi-attack The warden makes 2 Maul attacks and 1 Shield smash attack it can perform its Disintegrating Gaze instead of a Shield Smash if Recharged  

Maul Attack. Melee Weapon Attack: +12 to hit, reach 20 ft., one target. Hit: 18 (3d12+6) Bludgeoning damage.

Shield Smash. The Warden uses his shield to slam into the ground creating a Thunderwave centered on itself in a 10ft Radius.  Each creature in range must succeed on a DC 18 Strength or Dexterity saving throw (target's choice) or be pushed back 10ft and knocked prone and take 18 (3d12 + 8) Force damage plus 18  (3d12 + 8) Bludgeoning damage. On a successful save, the creature takes only half the damage, isn't knocked prone, and is pushed 5 feet out of the Wardens space into an unoccupied space of the creature's choice. If no unoccupied space is within range, the creature instead falls prone 

Disintegrating Gaze.  (Recharge 5–6) If the target is a creature, it must succeed on a DC 18 Dexterity saving throw or take 45 (10d8) force damage. If this damage reduces the creature to 0 hit points, its body becomes a pile of fine gray dust.

Full Cover (Recharge 5–6). As a reaction the Warden can impose full cover using its shield from all frontal attacks. 

The Adamantine Warden has 1 legendary action per round 

Necrotic Blast.  The warden targets a creature it can see. The targeted creature must make a DC 18 Constitution saving throw, taking 36 (8d8) necrotic damage on a failed save, or half as much damage on a successful one