Immutable. The Sentinel is immune to any spell or effect that would alter its form. 

Lightning Absorption. Whenever the Sentinel would be subject to Lightning damage, it instead takes no damage and regains hit points equal to the damage dealt.

Magic Resistance. The Adamantine Sentinel has advantage on saving throws against magical effects.

Magic Weapons. The Adamantine Sentinel's weapon attacks are magical.

Multiattack. The Sentinel makes two weapon attacks.

Great Sword. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 52 (8d10 + 8) thunder damage. 

Thunder Whip. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 45 (6d10 + 12) thunder damage.

Force Burst (Recharge 6). The Adamantine Golem creates an advancing sphere of force centred on the Adamantine. The sphere has a 15ft radius. Each creature inside the sphere must make a Strength saving throw (DC 24) or be pushed to the edge of the sphere and take 63 (14d8) force damage, half as much on a successful save. On a successful save, the creature is not pushed.

Planar Preservation. As a reaction to any creature or object attempting to pass through the entrance provided by the spell Genesis, the Sentinel can seal the entrance to prevent any passage. This can be re-opened by the Sentinel or the caster, and the entrance becomes open once the Sentinel is killed.