Ambusher. In the first round of a combat, the bandit captain has advantage on attack rolls against any creature it surprised. 

Pack Tactics. The bandit captain has advantage on an attack roll against a creature if at least one of the captain's allies is within 5 feet of the creature and the ally isn't incapacitated

Polearm Master. When the bandit captain takes the Attack action and attack with only a staff it you 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 d4, and it deals bludgeoning damage. While you are wielding a glaive, halberd, pike, quarterstaff, or spear, other creatures provoke an opportunity attack from you when they enter your reach.

Sentinel. When the bandit captain 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 captain even if they take the Dash action before leaving its reach. When a creature within 5 feet of the bandit captain makes an attack against a target other than the captain (and that target doesn't have this feat), the captain can use its reaction to make a melee weapon attack against the attacking creature.

War Caster. The bandit captain has practiced casting spells in the midst of combat, learning vital techniques.  The bandit has advantage on Constitution saving throws that it makes to maintain its concentration on a spell when it takes damage. The bandit captain can perform the somatic components of spells even when it has weapons or a shield in one or both hands. When a hostile creature's movement provokes an opportunity attack from the bandit captain, it can use its reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

Army Arcana. When the bandit captain casts a spell that causes damage or that forces other creatures to make a saving throw, it can choose itself and any number of allies to be immune to the damage caused by the spell and to succeed on the required saving throw.

Spellcasting. The bandit captain is a 9th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 15, +7 to hit with spell attacks). The mage has the following wizard spells prepared:

Cantrips (at will, 5th level): fire bolt, mind sliver, ray of frost, vicious mockery.

1st level (4 slots): command, hex, ice knife, shield

2nd level (3 slots): spiritual weapon, hold person, shatter

3rd level (3 slots): counterspell, fireball, enemies abound

4th level (3 slots): elemental bane, ice storm

5th level (1 slot): cone of cold

Multiattack. The bandit captain can make two melee attacks.

Staff of Shiver. Melee Weapon Attack: +4 to hit, reach 5 ft. or reach 10 ft. (two hands), one target. Hit: 4 (1d6 + 1) bludgeoning damage, or 5 (1d8 + 1) bludgeoning damage if used with two hands, plus 8 (2d8) cold damage.

While wielding this staff, any spells that deal cold damage receive a +1 to the attack roll and an addition 4 (1d8) cold damage to the damage roll. If a creature takes cold damage from being hit, by a melee attack, with this staff it has its movement speed reduced by 10 ft., can't take reactions, and can't take any action that requires a verbal component, for one round.

Dagger. Melee or Ranged Weapon Attack: +5 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 4 (1d4 + 2) piercing damage.

Parry. The captain adds 2 to its AC against one melee attack that would hit it. To do so, the captain must see the attacker and be wielding a melee weapon.