Castellan

Legendary Resistance (1/day): If the Castellan fails a saving throw, it can choose to succeed instead.

Vicious Strike: Once per turn, the Castellan can deal an extra 20 (5d6) damage to a creature it hits with a weapon attack.

War Stratagem: Once per turn, when the Castellan successfully hits a creature with a melee attack it can call upon an ally to make one ranged or melee attack against the attacked creature.

Spellcasting. The Castellan is a 10th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 16, +4 to hit with spell attacks). The Castellan has following wizard spells prepared:

Cantrips (at will): Fire Bolt, Prestidigitation, Message, Ray of Frost, Minor Illusion 

1st level (4 slots): Magic Missile, Shield, Alarm, Sleep 

2nd level (3 slots): Hold Person, Invisibility

3rd level (3 slots): Fireball, Slow

4th level (3 slots): Stoneskin, Greater Invisibility, Fire Shield

5th level (2 slots): Cloudkill, Cone of Cold

Multiattack: The Castellan can make two attacks per turn

Longsword: Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 4) slashing damage. 

Kick: Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 3 (1d4 + 4) bludgeoning damage. If the target is a medium sized or smaller creature, it must make a DC 15 Strength saving throw or be knocked prone.

Javelin: Ranged Weapon Attack: +4 to hit, range 30/120 ft., one target. Hit: 5 (1d6 + 4) piercing damage.

Poisoned Throwing Knife: Ranged Weapon Attack: +4 to hit, range 15/60 ft., one target. Hit:  4 (1d4 + 4). If the target is a creature that isn't immune to poison or a construct, it must make a DC 14 Constitution saving throw. If the creature fails, it takes 5 (1d8) poison damage and is poisoned. If it succeeds it takes no poison damage and isn't poisoned.

Unyielding Fury (Recharges after a Short or Long rest): The Castellan calls out a hostile creature to it. The called out creature is then marked. Every attack made against the marked creature has advantage. The mark goes away when the targeted creature is incapacitated or unconscious. This action cannot be used more than twice per encounter.

Heroic Burst (Recharges after a Long rest): Deep reserves of energy fill the Castellan during their time of need. Once activated, the Castellan takes an extra turn after this one. After the end of their next turn, the Castellan gains two levels of exhaustion. This action can only be used once every 24 hours.

Parry: The Castellan adds +2 AC against a melee attack that would normally hit it. To do so, the Castellan must see the attacker and be wielding a melee weapon.

Missile Deflect: The Castellan adds +1 AC against a ranged attack that would normally hit it. To do so, the Castellan must see the attacker and be wielding a shield. If the ranged attack misses, the Castellan can roll 1d6. On a 1 to 5, the Castellan is unaffected. On a 6, the Castellan redirects the missile back at the attacker as though it had originated from the Castellan.

The Castellan can take 2 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The Castellan regains spent legendary actions at the start of its turn.

Riposte: The Castellan makes one longsword attack against a creature that attacked it within 5ft. of it

Shift: The Castellan moves up to half its speed in any direction. This movement doesn't trigger opportunity attacks.