How to determine the proper CR for a Necromancer & Animated Dead

Question

Given the low CR of skeletons and zombies, I have attempted to create a reasonably low level Necromancer villain for my adventurers to thwart. While I am aware that monster creation involves playtesting for accuracy, I would still like to attempt to tune this as finely as possible in the generation stage.

Having already cast Animate Dead before the adventures arrive, the Necromancer has a bonus action available to command the undead under his/her command. Since there is no indication that the animated dead would act independently of being commanded, would I include the relevant statistics for the animated dead as part of the CR calculations for the Necromancer?

Would the HP of the animated dead count toward his/her Defensive CR and would the damage of the animated dead count toward his/her Offensive CR?

Related question in D&D 3.5

Answer 1

Calculate CR for the Necromancer and undead separately then use encounter adjusted experience for the group

In 5e CR is used to determine if a creature is appropriate with characters of a given level based on its damage/round and defenses. Creatures with a CR above the PCs level risk overwhelming them either by killing them instantly or soaking up damage beyond what the PCs resources can dish out.

While a Necromancer with a bunch of undead is going to do more damage than a Necromancer alone it isn't going to do this damage as a couple huge attacks like one high CR monster would. Similarly as the Necromancer's group loses HP it will lose creatures unlike a high CR monster suffering critical existence failure. Due to these differences the right way to stat this is by determining the CR of the necromancer and then determining the difficulty of an encounter with them and the group of undead.

Remember the undead will be available when calculating the Necromancer's CR

My experience has been that determining offensive CR for a spellcaster is tricky as a spell's effect often varies with how many allies they have. For example your Necromancer could cast Bless which would increase their groups expected damage by a hard to calculate amount. In your case since you know there will be 4 skeleton/zombies you can get a more accurate offensive CR by including their presence in your calculations when determining how effective the Necromancer's spells will be.

Answer 2

For better accuracy, treat them as separate monsters in the encounter

You should just treat the skeletons as separate monsters for the fight. Sure, the necromancer will be missing his bonus actions, and might have a slightly lower damage output as a result. Still, if your real interest is gauging the difficulty of the fight at the table, you would be better served by treating the undead separately.

As an example, I used the Kobold Fight Club to estimate the difficulty of an encounter with a mage from the MM, and a mage with 4 skeletons. For a group at level 5, the mage alone is a medium encounter, but the mage with 4 skeletons is a deadly encounter.