These may need tweaking, I've modified them for this campaign and added some of my own special abilities to these monsters. I've also upped Hit Points to make the challenge harder and hopefully get some party members to be able to play as Super Zombies.
—————————————————————-

Zombies:

Hit Points: 22 (Heavily Armored Zombies have 44 Hit Points)

Speed: 20ft

Regular Stats:
Str: 13(+1) For heavily armored: 15(+2) Dex: 6(-1)
Con: 16(+3) Int: 3(-4) Wis: 6(-1) Cha: 5(-3)
For heavily armored: 10(+0)

Other:
Saving Throws: WIS (+0) (Heavily Armored is STR(+2)
Damage Immunities: Poison (heavily armored zombies have resistance to slashing, piercing or bludgeoning damage from non magical attacks)
Condition Immunities: Poisoned
Senses: Darkvision 60ft, Passive Perception: 8
Languages: Understands the languages it knew in life but can't speak

Weapons: Heavily armored undead have a short sword

Special Abilities:
Undead Fortitude: If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success the zombie drops to 1 hit point instead.

Attacks:

Slam: Melee Weapon Attack: +3 to hit, reach 5ft., one target, Hit: 3d4+1 bludgeoning damage

Heavily armored can make an attack with their sword

————————————————————

Super Zombies:

Hit Points: 180
Speed: 35ft

Regular Stats:
Str: 18(+3) Dex: 10(+0) Con: 17(+2) Int: 11(+1)
Wis:10(+0) Cha: 19(+4)

Other:
Saving Throws: Charisma (+3)
Damage Immunities: Poison, Necrotic
Condition Immunities: Poisoned, blinded, deafened
Senses: Darkvision 60ft
Languages: Can understand any it spoke in life but cannot speak itself

Weapons: Whatever the character had when they died

Special Abilities:

Undead Fortitude: If damage reduces the super zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success the zombie drops to 10 hit points instead.

Intimidating Presence: When a super zombie enters the battlefield all party members must roll a Constitution saving throw with a DC of 16- upon a failed roll they suffer the Terror condition- they become paralyzed for a single turn and then must roll another Constitution saving throw in order to cure the condition. They cannot make an attack and can only move away from you while Terrified.

Allies in Death: If the Undead Beholder is on the battlefield the Super Zombie(s) gains a +4 to its Dexterity and Intelligence scores along with gaining an extra 10 Hit Points.

Attacks:

You may attack with the weapon you had when you were alive, and add 1d12 to that damage.

Slam: Melee Weapon Attack: +3 to hit, reach 5ft., one target, Hit: 3d12+5 bludgeoning damage

—————————————————————

Undead Hellhounds:

Hit Points: 45
Speed: 50ft.

Regular Stats:
Str: 17(+3) Dex: 12(+1) Con: 14(+2) Int: 5(-2) Wis: 13(+1) Cha: 6 (-2)
Other:

Skills: Perception +5
Damage Immunities: Fire, poison
Senses: Darkvision 60ft, Passive Perception 15
Languages: Understands infernal but can't speak it

Special Abilities:

Undead Fortitude: If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success the undead hellhound drops to 5 hit points instead.

Allies in Death: If a Super Zombie is on the battlefield the Undead Hellhound(s) gain a +2 to their attack rolls along with gaining an extra 5 Hit Points.

Keen Hearing and Smell: The hound has advantage on Wisdom(Perception) checks that rely on hearing or smell.

Pack Tactics: The hound has advantage on an attack roll against a creature if at least one of the hound's allies is within 5ft of the creature.

Attacks:

Bite: Melee weapon attack: +5 to hit, reach 5ft, one target. Hit: 7 plus (1d8+3) piercing damage

Fire Breath (Recharge 5-6): The hound exhales fire in a 15-foot cone. Each creature in the area must make a DC 12 Dexterity saving throw, taking 21 (6d6) fire damage on a failed save or half as much on a successful one.

___________________________________________

Undead Beholder:

Hit Points: 150
Speed: Fly (hover) 20ft

Regular Stats:
Str: 10(+0) Dex: 15 (+2) Con: 18(+3) Int: 16(+3)
Wis:16(+3) Cha: 19(+4)

Other:

Saving Throws: Int (+8) Wis (+7) Cha (+8)
Condition Immunities: Prone, deafened, poisoned
Senses: Darkvision 120ft., Passive Perception 20
Languages: Can speak broken phrases of Deepspeech and Undercommon

Weapons: None

Special Abilities:

Undead Fortitude: If damage reduces the undead beholder to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success the zombie drops to 25 hit points instead.

Intimidating Presence: When a super zombie enters the battlefield all party members must roll a Constitution saving throw with a DC of 16- upon a failed roll they suffer the Terror condition- they become paralyzed for a single turn and then must roll another Constitution saving throw in order to cure the condition. They cannot make an attack and can only move away from you while Terrified.

Allies in Death: If there are any Super Zombies on the battlefield the Zombie Beholder gains a +4 to its strength and dexterity scores.

Antimagic Cone: The Beholder's central eye creates an area of antimagic, as an antimagic field spell, in a 150ft cone. At the start of each of its turns the Beholder decides what direction the cone faces and if it is active. The area works against the Beholder's own eye rays.

Attacks:

Bite: Melee Weapon Attack: +5 to hit, reach 5ft., one target. Hit: One target, 14 (4d6) of pircing damage.

Eye Rays: The Beholder shoots three eye rays at random (roll for them, rerolling duplicates), choosing one, two or three targets it can see within a 120ft radius.

Charm Ray: The targeted creature must succeed on a DC 16 Wisdom saving throw or be charmed by the beholder for 1 hour, or until the beholder harms the creature.

Paralyzing Ray: The targeted creature must succeed on a DC 16 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Fear Ray: The targeted creature must succeed on a DC 16 Wisdom saving throw or be frightened for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Slowing Ray: The targeted creature must succeed on a DC 16 Dexterity saving throw. On a failed save, the target's speed is halved for 1 minute. In addition, the creature can't take reactions, and it can take either an action or a bonus action on its turn, not both. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
Enervation Ray: The targeted creature must make a DC 16 Constitution saving throw, taking 36 (8d8) necrotic damage on a failed save, or half as much damage on a successful one.

Telekinetic Ray: If the target is a creature, it must succeed on a DC 16 Strength saving throw or the beholder moves it up to 30 feet in any direction. It is restrained by the ray's telekinetic grip until the start of the beholder's next turn or until the beholder is incapacitated. If the target is an object weighing 300 pounds or less that isn't being worn or carried, it is moved up to 30 feet in any direction. The beholder can also exert fine control on objects with this ray, such as manipulating a simple tool or opening a door or a container.

Sleep Ray: The targeted creature must succeed on a DC 16 Wisdom saving throw or fall asleep and remain unconscious for 1 minute. The target awakens if it takes damage or another creature takes an action to wake it. This ray has no effect on constructs and undead.
Petrification Ray. The targeted creature must make a DC 16 Dexterity saving throw. On a failed save, the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified until freed by the greater restoration spell or other magic.

Disintegration Ray: If the target is a creature, it must succeed on a DC 16 Dexterity saving throw or take 45 (10d8) force damage. If this damage reduces the creature to 0 hit points, its body becomes a pile of fine gray dust. If the target is a Large or smaller non-magical object or creation of magical force, it is disintegrated without a saving throw. If the target is a Huge or larger object or creation of magical force, this ray disintegrates a 10-foot cube of it.

Death Ray: The targeted creature must succeed on a DC 16 Dexterity saving throw or take 55 (10d10) necrotic damage. The target dies if the ray reduces it to 0 hit points.

——————————————————————