Dr. Bones' Undead Amalgamation

You command any number of undead (minimum of 4) you control at the time of casting to rush into one another, and piece together their bodies. The action is very noisy, and disturbing. The Undead Amalgamation remains in-tact for 10 minutes (or longer if casted using a higher spell-slot), after which the undead that made it up separate, and any HP that was lost is evenly split among the undead. If it does not have enough hit points to give each undead at least 1hp, then you choose which type of undead is destroyed until you have reached the appropriate number of hp per undead unit. 
(If you would lose control over any number of undead that make up the Undead Amalgamation, their timer is suspended until Dr. Bones' Undead Amalgamation ends.)

—The Undead Amalgamation still counts as the number of Undead used to make it—and requires that many iterations of reasserting control to maintain your command over it.
—You may only have one Undead Amalgamation active at a time.
—Regardless of how long, or how many undead you use, you will always lose at least half of the undead that is used to make the Undead Amalgamation. (EX. Use 70 Zombies, and 30 Skeletons: Get 37 Zombies, and 15 Skeletons back when the spell ends.)
—The Undead Amalgamation Rolls its own initiative.

Initial effect upon casting: 
Creatures that are smaller than the Undead Amalgamation (according to how many undead you are using) and can see or hear the Undead Amalgamation that are not Undead, Constructs, or that are not Necromancy Wizards, Oath Breaker Paladins, Death Domain Clerics, Ranger's with their preferred enemy set as undead, (or whatever else your DM might agree to) have to make a Wisdom saving throw or become frightened by the Undead Amalgamation for as long as they can see or hear it. Creatures that can both see and hear the Undead Amalgamation have disadvantage on the initial saving throw. 
Creatures that are frightened by the Undead Amalgamation spend their turn running as far away from the Undead Amalgamation; they can make the saving throw again at the end of their turn. Creatures gain advantage on the saving throw if they can no longer hear or see the Undead Amalgamation.
Creatures that can't be frightened are immune to this.

Base Undead Amalgamation Stats:
| STR: 13 | DEX: 10  | CON: 15 | INT: 4 | WIS: 7 | CHA: 5 |
Senses: 10 Passive Perception; Darkvision (60ft)
Condition Immunities: Exhaustion, Poisoned, Charmed, Frightened.
Damage Vulnerabilities: Radiant.
Damage Resistance: Necrotic.
Damage Immunities: Poison.
Languages: all that they knew in life, but it cannot speak any.
CR: it depends.

Each Skeleton adds:
— +1 to the overall Dexterity score [Max of 30]
— -.5 to the overall Strength Score [Min. 3]
— +13 Max HP (2D8+4)
Each Zombie adds: 
— +1 to the overall Strength score [Max of 30]
— -.5 to the overall Dexterity score [Min. of 3]
— +22 Max HP (3D8+9)

At 5 Undead in total:
—Powerful build: is medium sized, but for the purposes of lifting, pushing, pulling they count as one size larger.
—Save DC = 12
At 5 Skeltons:
—+5ft of movement (Stacks for every set of 5 skeletons the Undead amalgamation has; Max of 100ft)
—Multi-Attack (action): [(2X)Short-sword: 2D6+STR/DEX (+STR/DEX+2 to hit), (1X)Short-bow: 1d6+DEX (+DEX+2 to hit) (Range: 80/320ft)]
At 5 Zombies:
— -5ft of movement (Stacks for every set of 5 zombies the Undead amalgamation has; the undead amalgamation can be reduced to 5ft of movement, but not 0ft in this manner.)
— -1AC (Stacks for every set of 5 zombies the Undead Amalgamation has; the Undead Amalgamation can be reduced to an AC of 8, but no lower in this manner)
—Reach: +5ft (Stacks for every set of 5 zombies the Undead Amalgamation has; MAX of 30ft)
—Heavy Slam (Action): (Range: 15ft cone) 3d12+STR (Creatures caught in the range make a Strength Saving Throw against the Undead Amalgamation's Save DC being knocked prone on a failed save, and taking only half damage and not knocked prone on a successful save.)

At 15 Undead in total:
—Size increases to Large
—Immune to Prone, and Paralyzed conditions.
—Darkvision increases to 120ft.
—Save DC increases to 16
At 15 Skeletons:
—Dervish (Action): [(5X)Short-sword: 2D6+STR/DEX (STR/DEX+2 to hit), (5X)Short-bow 1D6+DEX (+DEX+2 to hit) (Range: 80/320ft)]
At 15 Zombies:
—Grotesque Leap (Bonus Action): The Undead Amalgamation leaps into the 50ft into the air (it does take fall damage from doing this), and slams into the ground. Creatures and objects within 30ft radius of the Undead Amalgamation when it lands must a make Constitution saving throw against the Undead Amalgamation's Save DC or take 5D8 Necrotic and 3D12 Poison Damage, or half as much on a successful save. Creatures that the Undead Amalgamation would land on make a Dexterity Saving Throw (with advantage if they saw the Undead Amalgamation jump into the air) as well, and take 5D10 Bludgeoning damage, and are knocked prone, or take no damage on a successful save, and move to a safe space next to where the Undead Amalgamation would land.

At 45 Undead in total:
—Size increases to Huge
—Immune to Grappled and Restrained conditions.
—Aura of Death: Creatures and objects within 10ft of the Undead Amalgamation take 10 necrotic damage (causing structures, plant life, etc. to crumble and decay), and Creatures within 30ft. take 5 necrotic damage.
—Giant Weapons: If you are capable of doing so, or procuring the proper workforce to forge such a weapon, the Undead Amalgamation may wield a Giant Weapon. The weapon can be whatever you design, and will be a much larger version of a martial (not simple) weapon. Dealing 5X the normal damage of the base weapon. You may also apply magical buffs (this may increase gold cost). To craft a weapon such as this, you require 100X the gold required to purchase on page 149 of the PHB. 
—Giant Armor: If this is something that you desire, know that you cannot make light armor. Medium armor has a strength requirement of 16. Likewise, the strength requirement of heavy armor is increased by 10 [EX. Chain mail: 13STR pre-req.; Giant Chain mail: 23STR pre-req.] You may also apply magical buffs, (may increase gold cost). Furthermore, this too costs 100X the gold required to purchase on page 145 of the PHB. The armor gives the same benefits as it normally would.
—Save DC increases to 20
At 45 Skeletons:
—Concentrated Barrage (Reaction Buff): The Undead Amalgamation can take a number of reactions, equal to its Dexterity Modifier. It can only use these reaction to target creatures that move at least 15ft within it's short bow range [80/320]. It rolls 2D6+DEX (+DEX+2 to hit). It regains 3 reactions at the end of each of its turns.
At 45 Zombies:
—Death-Bound-Battle-Grounds (Held Action—1 minute to set up): The Undead Amalgamation spreads its form out across the battlefield, covering a 50ft square (or a minimum of a 20ft square) as if a bloody battle had taken place, and remains inert until you decide to trigger the action. (The smaller the radius that is used, the closer the bodies are piled together.) When triggered, creatures that could move outside of the radius with half of their movement make a Dexterity saving throw against the Undead Amalgamation's Save DC. On a success, they run out of the radius by the quickest means possible. Creatures that fail, or are unable to make it out of range are restrained, as the Undead Amalgamation forcibly reforms itself. Creatures caught inside take 5D8Necrotic, 10D10 Bludgeoning. Creatures that cannot be restrained still take the damage, but are immediately ejected from the Undead Amalgamation. (Incorporeal creatures such as Ghosts take only half damage, or none on a successful save.)
A successful Investigation, or Perception check against the Undead Amalgamation's Save DC reveals that the undead in the area are indeed animated, and are just playing dead. However, it does not reveal that they are all part of the same undead. If an ability such as the Paladin's Divine Sense is used, it reveals that the Undead Amalgamation that spans the 50ft square (or a minimum of a 20ft square) is all the same undead. 
Improved Grotesque Leap (Bonus Action): The Undead Amalgamation leaps into 100ft into the air (it does take fall damage from doing this), and slams into the ground. Creatures and objects within 45ft radius of the Undead Amalgamation when it lands must a make Constitution saving throw against the Undead Amalgamation's Save DC or take 10D8 Necrotic and 6D12 Poison Damage, or half as much on a successful save. Creatures that the Undead Amalgamation would land on make a Dexterity Saving Throw (with advantage if they saw the Undead Amalgamation jump into the air) as well, and take 10D10 Bludgeoning damage, and are knocked prone, or take no damage on a successful save, and move to a safe space next to where the Undead Amalgamation would land. 

At 135+ Undead in total:
—Size increases to Gargantuan
—Greater Aura of Death: Creatures and objects within 10ft of the Undead Amalgamation take 30 necrotic damage (causing structures, plant life, etc. to disintegrate, and rot into nothingness), Creatures and objects within 30ft. take 10 necrotic damage, and Creatures within 60ft take 5 necrotic damage.
—Immune to the Stunned condition.
—Greater Giant Weapons: You may, if you so desire, create a weapon that is so large that it causes tremors when it strikes the ground. To craft this Greater Giant Weapon, you follow the same rules as before: must be a martial weapon, and you've got to fork over a ton of gold (or find a suitable weapon somewhere). To craft a Greater Giant weapon costs 300X the gold cost on page 149 of the PHB. This weapon deals 10X the normal damage of the normal weapon. This weapon causes each creature that is hit by it that is not Gargantuan size or larger to make a strength saving throw, or be knocked prone, and pushed 30ft back. Creatures within 15ft of the creature targeted by an attack from this weapon also makes a strength saving throw or is knocked prone, and pushed 10ft away from where the target was. 
—Greater Giant Armor: Just as before, you may create really big armor. It can be magical, it'll just cost more (probably). It costs 300X more gold to buy than what it says on page 145 of the PHB. The Strength requirement for Medium armor is 20, and all heavy armor is 30. Go nuts with your big undead knight.
—Darkvision increases to 240ft.
—Save DC increases to 24.
At 135 Skeletons:
—Torrent of Arrows and Blades (Action/Recharge(6)1D6): Multi-Attack [(20X)Short-sword: 2D6+STR/DEX (STR/DEX+2 to hit), (15X)Short-bow 1D6+DEX (+DEX+2 to hit) (Range: 80/320ft)] 
At 135 Zombies:
—Grand Finale: Creatures and Objects within 120ft of the Undead Amalgamation makes a Constitution Saving throw. On a failed save, and if they are within 60ft of the Undead Amalgamation they take 20D8+60 Necrotic Damage, and 15D12+40 Poison Damage, or half as much on a successful save. Creatures that are 65ft and beyond take 20D8 Necrotic Damage, and 15D12 Poison Damage, or half as much on a successful save. [When Grand Finale is used, 100 Zombies that make up the Undead Amalgamation are liquified and turned into necrotic energy. This reduces the Max HP, size, stats, and so forth accordingly. Any Greater Giant Armor that the Undead Amalgamation was wearing is damaged, and must be repaired.
—Massive Slam (Action): (Range: 60ft cone) 10d12+STR (Creatures caught in the range make a Strength Saving Throw against the Undead Amalgamation's Save DC being knocked prone on a failed save, and taking only half damage and not knocked prone on a successful save.)

Dr. Bones' Undead Amalgamation spell can be upcasted to increase the time the Undead Amalgamation lasts:
If casted at 7th level, it remains for 1 hour; if casted at 8th level, it remains for 8 hours; if casted at 9th level, it remains for 24 hours.