Defender. All attacks made against the Sentinel have disadvantage.

In addition whenever the Sentinel reduses a creature to 0 hp with a melee weapon attack, it can use its reaction to make another melee attack with the same weapon against a different creature within reach.

Regeneration. The Sentinel regains 40 hp at the start of each of its turns if it has at least 1 hitpoint. The Sentinel dies only if it starts it turn with 0 hp and doesn't regenerate.

Never ending servitude. If the Senitinel was redused to 0 hp it will feign death by lying dormant for 2 hours before begining reassembling itself, a process taking 24 hours.

Sige Monster. The Sentinel deals double damage to objects and structures.

Defender. All attacks made against the Sentinel have disadvantage.

Monstrosity. At the start of the Sentinels turn if it took damage roll a d10, on a 10 the Sentinel becomes monsterus until the end of its next long rest.
while monsterus the sentinel gets a +10 bonus to attack and damage rolls, it loses the benefists of defender and regeneration.
In addition whenever the Sentinel reduses a creature to 0 hp with a melee weapon attack, it can use its reaction to make another melee attack with the same weapon against a different creature within reach.

Regeneration. The Sentinel regains 40 hp at the start of each of its turns if it has at least 1 hitpoint. The Sentinel dies only if it starts it turn with 0 hp and doesn't regenerate.

Never ending servitude. If the Senitinel was redused to 0 hp it will feign death by lying dormant for 2 hours before begining reassembling itself, a process taking 24 hours.

Sige Monster. The Sentinel deals double damage to objects and structures.

Actions

Multiattack. The Sentinel makes 3 attacks.

Spear. Melee weapon attack +12 to hit, reach 15 feet, one target. Hit: 24 (3d6+8) Piercing damage.

Longsword. Melee weapon attack +12 to hit, reach 10 feet, one target. Hit: 21 (3d8+8) Slashing damage, or Hit 24 (3d10+8) Slashing damage, if wielded in both hands

Stomp. Melee weapon attack +12 to hit, reach 15 feet, one creature within reach that is prone. Hit: 30 (4d10+8) Bludgeoning damage.

Stunning Burst. (recharge 6) The Sentinel creates a grating, thunderous sound, in a 15 foot radious sphere around it. Each creature within must make a dc 18 constitution saving throw , taking 63 (14d8) thunder damage and falling prone on a failed save, or half as much on a succesfull save. Creatures that save do not fall prone

Multiattack. The Sentinel makes 3 attacks.

Spear. Melee weapon attack [+12] to hit, reach 15 feet, one target. Hit: 24 ([3d6+8]) Piercing damage.

Longsword. Melee weapon attack [+12] to hit, reach 10 feet, one target. Hit: 21 ([3d8+8]) Slashing damage, or Hit 24 ([3d10+8]) Slashing damage, if wielded in both hands

Stomp. Melee weapon attack [+12] to hit, reach 15 feet, one creature within reach that is prone. Hit: 30 (4d10+8) Bludgeoning damage.

Stunning Burst. (recharge 6) The Sentinel creates a grating, thunderous sound, in a 15 foot radious sphere around it. Each creature within must make a dc 18 constitution saving throw , taking 63 (14d8) thunder damage and falling prone on a failed save, or half as much on a succesfull save. Creatures that save do not fall prone