Dr. Bones' City of Walking Corpses

Crafting the Ruby Incrusted Pot for the City of Walking Corpses:
—You place the aborted fetus of a fawn inside of the ruby incrusted pot, and then pour the platinum dust inside of the pot. Afterwards, you seal the pot whilst muttering foul and dark incantations—this requires you to make a wizards sigil (really, just whatever symbol best represents you is what is on the sigil), and then you cork the pot with it. You must also write your name, or an closely tied alias that most people know you by—don't ask why, just do it—this name must be written in your blood (or the blood of your closest ally, if you lack blood). 

—You will know the spell was performed successfully by the soft but echoing and pitiable bleating emanating from the ruby incrusted pot, (out to a range of 5,000ft; the sound does change in tone at 100ft (soft and painful bleats every 6 seconds/every round), 500ft (sharp and in bursts of three every 12 seconds/every 2 rounds), 1,000ft (depressed and echoing bleats every 30 seconds/every 5 rounds), 2,000ft (agonized and loud bleating every 1 minute), 3,000ft (desperate, and confused bleating every 5 minutes), and 5,000ft (dying and exasperated bleating occurring every 10 minutes).

Outside interference with the ruby incrusted pot:
—To remove the sigil requires a Strength (athletics) check against your spell save DC+10. If that seal is ever removed, or if the spell is dispelled (via Remove Curse, or the Wish spell) then the spell ends and all undead within 60ft of the pot are turned for 1 minute (the origin point is where the pot).
—1If the pot is destroyed [It has an AC equal to your spell save DC, and has HP=40; it is immune to all conditions and damage types except for Force.]
—If an ability that would target blood is used on the pot, then the creature responsible must make a Constitution Saving throw, or spend their turns vomiting blood for the next minute. Every turn they take 1D4 Psychic Damage, and 2D6 Necrotic Damage, and are incapacitated as they vomit. The blood vomiting can be stopped by casting Heal, Greater Restoration (at 7th level or higher), or the Wish Spell. The individual who caused this would then be able to tell the exact location of the caster who created this ruby incrusted pot for 1 hour if they are on the same plane of existence, or they would be able to tell which plane of existence they are on. The same can be said of the spellcaster—if someone manipulates the blood of the aborted fawn inside of the pot, then the spellcaster is struck with visions of the entirety of the area effected by the ruby incrusted pot [5,000ft]—however, they also suffer whatever effect would have effected the ruby incrusted pot. They fail any save required, and if there is an attack roll, it auto hits.

When this spell is successfully casted, the following occurs:
—Undead within 5,000ft of the pot gain an intelligence score equal to half of your intelligence rounded down, (this does not apply to undead that have an intelligence score higher than what they would receive.) (This effect persists for a number of years equal to 1/4 your intelligence modifier, after which the undead is sent into a frenzy and attacks all living creatures it can see until it renters the effect of this spell.)
—Undead within 3,000ft. of the pot gain the ability to speak all languages that the spellcaster knows—they do not speak any that they knew in life, but whenever the spell caster casts a spell such as Comprehend Language, or Tongues, all of the undead that under this effect are also effected by those spells. (This effect persists for a number of years equal to 1/4 your intelligence modifier. If this time ever runs out, they shout out the name (or alias) of the spell caster who's name/alias is written in the pot—then they enter the aforementioned frenzy and can no longer speak.)
—Undead that maintain a distance of at least 2,000ft share a simi-collective consciousness. (In layman's terms: they know when one of them is being attacked, talked to, or is doing something. They don't learn specifics, but they have a general idea of what is going on—it is not an overwhelming experience for them, but the silence of being outside of the range can sometimes cause the undead to enter a frenzy if they are forcibly removed from the range.)
—Undead that remain within 1,000ft of the pot regenerate their natural form, and appear more living than they otherwise would. This effect occurs only if they remain within that distance for a total time of 1 year; the change slowly takes place as time progresses, and they still count as undead—though they do not look it. (This effect persists for six months without decay if the undead ever leaves the range, and after those initial six months the decay progressively grows worse until they become what they originally were. They don't go crazy if this happens, they just look like what they are now.)
—Undead that have remained with 500ft of the ruby incrusted pot gain your alignment, and if they've remained for at least a year then they regain their previous abilities [they essentially become living creatures again—save for their creature type still being undead), and false memories of a happy life, and a deep and devoted friendship with the spellcaster who created the ruby incrusted pot. (this persists as long as they remain within the effects of a ruby incrusted pot created by the spell caster—can be another one. If the undead leaves the range, then these memories remain for 1 month, and then the undead begins to remember more about their real life. After 6 months, they know that they had false memories implanted into their minds; however, if you treated them well, they may not hold it against you. Especially if you have been expressly interested in their well being, and proven to just want them to be happy. Even still, if they were originally the sort to seek retribution for any slight, your good intentions might not mean much to them. This can all be avoided by simply allowing the undead to have their original memories back—to do so requires 10 minutes per undead, and 100GP for creating a glyph on the back of the undead's head that restores their memories (this does not work on the living). If this is done, their loyalty and friendship cannot be guaranteed, and their original alignment is restored. 
—Undead that remain within 100ft of the pot regenerate 1/2 of the damage they've taken every round (restoring to full health when there is only 5HP or less remaining to heal). [This persists so long as they are within 100ft of the ruby incrusted pot.]

Creatures roll a Wisdom Save against your spell save DC every time they enter the following ranges, and if they fail the save then they suffer from the following as well...:
—At 5,000ft: [make the save every 10 minutes] [WIS SAVE] Or gain from 1 level of exhaustion.
—At 3,000ft: [make the save every 5 minutes] [WIS SAVE] Or suffer from Confusion (as if cast from the Confusion spell) for 1 minute.
—At 2,000ft: [make the save every 1 minute] [WIS SAVE] Or suffer from the Deafened and Stunned condition until the end of their next turn.
—At 1,000ft: [make the save every 30 seconds] [WIS SAVE] Or fall Unconscious for 1d4 rounds.
—At 500ft: [make the save every 12 seconds] [WIS SAVE] Or suffer from the frightened condition, and the source is the ruby incrusted pot. While frightened in this way, you also gain disadvantage on Charisma checks made to convince you to leave the area.
—At 100ft: [make the save every 6 seconds] [WIS SAVE] Or suffer from the Charmed condition until the end of their next turn. If the Creature is unable to be charmed, they instead take 3D12 Psychic Damage, or none on a successful save. A creature charmed this way views the undead around them as friendly, and is instructed to persuade their ally's not to harm any of the undead or the Ruby Incrusted Pot (if the creature knows of its existence).

To avoid this, a creature needs either a blessing, a charm (both found on pg. 228 in the DMG); though it recommended the DM creates their own if they wish to use this path.) Or, they could be given admittance by the creator of the Ruby Incrusted Pot; this admittance could be a sigil they carry with them into the area, an amulet, a piece of parchment, an utterance they make 8 hours to stave off the effects of the spell, they might also have specifically casted a wish spell to become immune to the effects (though this immunity does not last for more than 1 month).
Any of these options might also have been stolen. 
Regardless of the methods, every individual creature needs a way to stave off the effects for themselves, and the charms or amulets or what have you's cannot be shared.
Furthermore, if a creature becomes friends with one of the local undead, and that undead has their original memories, then they may find a way to sneak the creatures inside of the boundaries without having to worry about the debilitating effects of the spell. Such as... magic earplugs? I don't know, but make it something whimsical or silly. Like a cowbell they have to hit every time someone uses the word "the" or "at". 

DEVELOPER NOTE: *At any rate, this is a spell that isn't truly intended for players, but like, why not let them cast it if they have the resources and want to make a not-so-undead city. Could be fun.