Round 4: Hailstones rain down from the cloud. Each creature under the cloud takes 2d6 bludgeoning damage.
Round 5-10: Gusts and freezing rain assail the area under the cloud. The area becomes difficult terrain and is heavily obscured. Each creature there takes 1d6 cold damage. Ranged weapon attacks in the area are impossible. The wind and rain count as a severe distraction for the purposes of maintaining concentration on spells. Finally, gusts of strong wind (ranging from 20 to 50 miles per hour) automatically disperse fog, mists, and similar phenomena in the area whether mundane or magical.
A churning storm cloud forms, centred on a point you can see and spreading to a radius of 360 feet. Lightning flashes in the area, thunder booms, and strong winds roar. Each creature under the cloud (no more than 5,000 feet beneath the cloud) when it appears must make a Constitution saving throw. On a failed save, a creature takes 2d6 thunder damage and becomes deafened for 5 minutes.
Each round you maintain concentration on this spell, the storm produces additional effects on your turn.
Round 2: Acidic rain falls from the cloud. Each creature and object under the cloud takes 1d6 acid damage.
Round 3: You call six bolts of lightning from the cloud to strike six creatures or objects of your choice beneath the cloud. A given creature or object can't be struck by more than one bolt. A struck creature must make a Dexterity saving throw. The creature takes 10d6 lightning damage on a failed save, or half as much damage on a successful one.
Even while you hold the heart, the Undead aren’t directly under your control, and therefore don’t count against any limitations of Undead you can command.
The Undead remain animated until the spell ends. The spell ends if the heart is destroyed (AC 20, HP 10, immunity to necrotic, poison, and psychic damage) or targeted by a dispel magic spell cast as a 9th-level spell. When the spell ends, all Undead animated by it are instantly destroyed, crumpling into a heap. The spell also ends if every Undead created by it is destroyed.
When you cast this spell, you stab a black, ritualistically prepared Humanoid heart. The ichor from the heart infects the land with foul necromantic energies, causing up to one hundred piles of bones or corpses of Medium or Small Humanoids within a 1-mile radius to rise from the dead, becoming skeletons or zombies (the GM has the creatures’ game statistics). The Undead dig their way out of graves, smash their way out of mausoleums, and tear down cemetery gates.
The creature that is holding the heart can use its action to issue a general command to all Undead created by the spell, such as to destroy a town or hunt down a particular creature. Only one general command can be issued to the Undead every 24 hours. If not issued a command, the Undead indiscriminately attack living creatures, other than the creature that is holding the heart and creatures designated by it. Once given an order, the Undead continue to follow it to the best of their ability until the task is complete.