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!
Ok this might be ridiculous but you want LOTS of skeletons and I was thinking different scenarios. Let's assume you stand in a field of skeletons since you have raised tons of skeletons beforehand and they just can't run away. Now you want to control as many of them as possible
Scenario 1: 18,720 Skeletons
You are a level 20 wizard with unlimited amounts of Spell Scrolls (you need to be wizard or cleric to have access to the spell, otherwise you need to roll for success). A level 9 Animate Dead lets you control 13 Skeletons. The casting time of the spell is 1 minute and since your control lasts 24 hours until you need to cast it again, you can cast it a maximum of 1,440 times a day (if you don't need to eat or sleep etc). So 1,440 * 13 = 18,720 Skeletons under your command.
If you want to sleep, eat and command them you just subtract the minutes and you get your number of skeletons. That way you have your spell slots and LOTS of skeletons (but need to get your hands on a lot of spell scrolls).
Scenario 2: 390 Skeletons
Level 9 Divine Soul Sorcerer
Level 11 Warlock
Ring of Spell Storing (needs attunement)
Pearl of Power (needs attunement)
Rod of the Pact Keeper (needs attunement)
The divine soul sorcerer has access to Cleric spells, so you can learn Animate Dead (when you get your lvl 3 Spell Slot). At level 11 a Warlock has 3 level 5 Pact Spell slots which recover with a short rest. A level 5 Animate Dead let's you control 5 Skeletons; You can cast them 3 Times a short rest so you get 15 Skeletons with 3 minutes per short rest. With 22 short rests (66 casting minutes) you have 330 Skeletons and 54 minutes left of the day.
You add level 5 Animate Dead from your Ring of Spell Storing: 5 skeletons. Total of 335 Skeletons. (53 minutes left)
Pearl of Power to regain a level 3 spell slot which can be converted into 5 sorcery points.
And then you have your sorcerer spells also: 1 level 5 Spell (5 Skeletons) and you have 9 sorcery points + 8 points (created from your level 1 spells) + 9 points (from level 2 spells) + 15 points (from level 3) + 18 points (from level 4) = 59 sorcery points + the 5 from pearl of power convert = 64 points. So you can cast 9 more or your level 5 spells (45 Skeletons). So with your sorcerer class you can create additional 50 Skeletons.
Rod of the Pact Keeper lets you regain 1 warlock spell slots so one additional level 5 Animate Dead for 5 Skeletons.
Total of 390 Skeletons (42 minutes left). Again no sleeping and this time you use all your spell slots but you don't need unlimited number of spell scrolls. So you could do this every day (except maybe the Ring of Spell storing filling, which would take some of your spells and except maybe the exhaustion without sleeping)
Check this linked answer. But I'm not sure if their math is incorrect or mine.