Fantasy novels are rife with evil necromancers controlling hordes of undead...and they also make great BBEGs! But what if a player wanted to do the same thing?
I'd like to see how big of an army a 5e D&D character could raise and control. Well, I'm actually more interested in the controlling aspect, as creating undead is pretty straight forward in terms of numbers per day.
Let's assume this character is 20th level, can be of any published class or class combo, only wants to make an army of permanent skeletons (they smell better than zombies!), has a 20 for any relevant ability score, and has access to any published non-artifact magic item.
As an example, let's assume the character is a wizard and is using the Animate Dead spell which says
This spell creates an undead servant.
The creature is under your control for 24 hours, after which it stops obeying any command you've given it. To maintain control of the creature for another 24 hours, you must cast this spell on the creature again before the current 24-hour period ends. This use of the spell reasserts your control over up to four creatures you have animated with this spell, rather than animating a new one.
My math says this wizard would only be able to control 128 skeletons with this spell. This is a LONG way from anything resembling an army...and the wizard would have to burn every single spell slot of 3rd level or higher, every single day, just to keep those 128 in line.
So, let's get creative! Remember, we're looking for LOTS of skeletons, and ideally we wouldn't have to burn all our spell slots every day just to do it!