Armed Sentinel

Reactive Combat. The Armed Sentinel takes no actions on its turn and does not move.  The Armed Sentinel can take one reaction on each of its enemy's turns.

Sentinel. The armed sentinel has the following traits similar to the Sentinel feat.

• When the Armed 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 Armed Sentinel even if they take the Disengage action.
• When a creature within 15 feat of the Armed Sentinel makes an attack against a target other than the Armed Sentinel (and the target doesn't have this feat), the Armed Sentinel can use its reaction to make a melee weapon attack against the attacking creature.

Mage Slayer. The Armed Sentinel has the following traits similar to the Mage Slayer feat.

• When a creature within 15 feet of the Armed Sentinel casts a spell, the Armed Sentinel can use its reaction to make a melee weapon attack against that creature.
• When the Armed Sentinel damages a creature that is concentrating on a spell, that creature has disadvantage on the saving throw it makes to maintain its concentration.
• The Armed Sentinel has advantage on saving throws against spells cast by creatures within 15 feet of it.

Polearm Adept. While wielding its glaive, the Armed Sentinel can make an opportunity attack against a creature entering its range.

Spellcasting. The Armed Sentinel is a 18th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 16, +8 to hit with spell attacks). The Armed Sentinel has following spells prepared:
1st level (at will): Shield
3rd level (at will): Hellish-Rebuke, Counterspell

The Armed Sentinel takes no actions on its turns and can not move.

Sentinel's Glaive. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 21 (2d10+10) slashing damage.

Deflect Missile. In response to being hit by a ranged weapon attack, the Armed Sentinel deflects the missile.  The damage it takes from the attack is reduced by 1d10 + 12. If the damage is reduced to 0, the missile is destroyed in the process.