NOTE: I would never give this to a player. This is based on a spell by the same name on dandwiki.com with a few adjustments. 

The radius of the spell depends on how many wizards who made it through the end of the spellcasting process are still alive. 32 Wizards (40,000 miles) - 24 Wizards (30,000 miles) - 16 Wizards (20,000 miles) - 8 Wizards (10,000 miles) - 4 Wizards (5,000 miles) - 3 Wizards (2,500) - 2 Wizards (1,250) - 1 Wizard (625 miles)

A minimum of four wizards who have ninth level spell slots are needed to begin the spellcasting for this spell. The spellcasting is complex, forcing each wizard to make an Intelligence (Arcana) check at the dawn of each day during spellcasting, DC 18. On a success, the wizard understands what to do next for the spell. On a failure, the wizard fails to understand what to do next for the spell, and no longer can be a part of the spellcasting process. If this causes there to be less than four wizards remaining in the spellcasting process, then the following effects happen.

• All wizards who were involved in the spellcasting process gets pulled into Shadowfell

• Each of the dragon corpses reaminate as shadow dragons, and their hit point maximum are double their previous hit point maximum

The dragon corpses must be placed in the corners of a "sqaure," in the respective direction from the center of the "square." I.e. the red dragon was killed in a town northeast of the place where the spellcasting is. The red dragon's corpse is placed to the northeast of the center of the spellcasting process. At least four wizards must then hold a heart of a Shadow Dragon in their hands for the whole casting time, during which necrotic energy swirls around the casting site and dreadful despair overcomes those who will be within range, the feeling being more weak the farther away the creatures are. This does not affect the wizards taking part in the spellcasting process.While a wizard is being a part of the spellcasting process, it does not need to eat, drink, sleep, or excrete.

Once the spell starts, a rift to Shadowdell forms in the sky above the casting site, vomiting black fog, a completely dark shadow that covers the whole sky in a 10-mi radius. Each dawn, this radius doubles until the spell is complete, at which point it rapidly expands to the full range (the range varies based on how many wizards who made it through the end of the spellcasting process is still alive).

While the spell is still in progress, corpses take 1 minute to rise in undeath. Humanoids turn into zombies or skeletons, while other creatures turn into undead counterparts. i.e. dragons turn into dracoliches. The necrotic plague is still in effect. 

Within this area, corpses begin to rise constantly, one after another who may have been slain. Each of these undead creature will be a zombie or a skeleton. A necrotic plague also develops upon the appearance of the absolute darkness. Each creature within range must make a Constitution saving throw at dawn each day, DC 15. On a success, the creature is unaffected by the plague for that day. On a failure, the creature is inflicted the plague for 1d4 days, and the following effects apply.

• Every other hour, both the victim's hit point maximum and current hit points decrease by 1

• The victim must make a Constitution saving throw each dawn it is inflicted with the plague, on a failure, the victim takes 1d6 poison damage and 2d6 necrotic damage, on a success, the victim gains advantage on its next Constitution saving throw against this plague

At dawn of each day, each wizard who took part in the spellcasting of this spell and made it through the end of the spellcasting process must make a Constitution saving throw, DC 23. On a failed save, the wizard takes 15d10 necrotic damage. On a roll of 1, the wizard is pulled into Shadowfell. Undead wizards do not need to make the saving throw. The spell ends early if all wizards who was still involved in the spellcasting process at the end is dead.