Clockwork Sentinel

Immutable Form. The Clockwork Sentinel is immune to any spell or effect that would alter its form.
Magic Resistance. The Clockwork Sentinel has advantage on saving throws against spells and other magical effects.
Magic Weapons. The Clockwork Sentinel's weapon attacks are magical.
Death Burst. When the Clockwork Sentinel dies, it explodes. Each creature within 5 feet of it must make a DC 13 Dexterity saving throw, taking 14 (4d6) fire damage on a failed save, or half as much damage on a successful one.

Combat Programming. The Clockwork Sentinel has been programmed with the following specialized combative abilities.

Programmed Shield Mastery. 

• If the Clockwork Sentinel isn't incapacitated, it can add its shield's AC bonus to any Dexterity saving throw it makes against a spell or other harmful effect that targets only it.
• If the Clockwork Sentinel is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, it can use its reaction to take no damage if the Clockwork Sentinel succeeds on the saving throw, interposing its shield between itself and the source of the effect.

Programmed Polearm Mastery.

• When the Clockwork Sentinel takes the Attack action and attacks with only a glaive, halberd, quarterstaff, or spear, it can use a bonus action to make a melee attack with the opposite end of the weapon. This attack uses the same ability modifier as the primary attack. The weapon’s damage die for this attack is a d6, and it deals bludgeoning damage.
• While the Clockwork Sentinel is wielding a glaive, halberd, pike, quarterstaff, or spear, other creatures provoke an opportunity attack from the Clockwork Sentinel when they enter its reach.

Sentinel Programming. 

• When the Clockwork Sentinel hits a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn.
• Creatures provoke opportunity attacks from the Clockwork Sentinel even if they take the Disengage action before leaving the Clockwork Sentinel's reach.
• When a creature within 5 feet of the Clockwork Sentinel makes an attack against a target other than it (and that target doesn't have this feat), the Clockwork Sentinel can use its reaction to make a melee weapon attack against the attacking creature.

Multiattack. The Clockwork Sentinel makes two polearm attacks. 

Polearm. Melee Weapon Attack: +8 to hit, reach 15 ft. one target. Hit: 13 (2d8+5) piercing damage.

Programmed Shield Evasion. If the clockwork Sentinel succeeds on a DEX based save to only take half damage, it can use its reaction to take no damage instead.