I just thought I would take a break from my regular posting of expanding the number of creatures you can summon with the conjure spells in your Fifth Edition game and post a monster I made just for fun. How to make NPCs for 5e is a quesiton I don’t have a good answer for since examples in the MM do not coorelate to levels in the PHB and their stats are completely off from what a monster of that challenge level should be in the DMG. The archmage is a prime example, Its an 18th-level spellcaster, is a challenge 12 monster but the hit points and armor class of a challenge 2 monster. Let me repeat that: the 18th level NPC has hit points and an armor class of a monster 16 levels below what it should be.
So how do I adjust that when I make an NPC? Short answer: The level of the NPC is it’s challenge rating. That 18th level archmage should be a challenge 18 NPC. How do I have it meet the stats of a challenge 18 monster? It gets as many hit dice as needed to be in that range. Think of it as that the NPC also has levels of aristocrat, commoner, or warrior—to use a 3.5/Pathfinder-ism. From there I make option choices to get all the other numbers where they should be.
Take the halfling warlock skeleton below as an example. I settled on a level 3 warlock, so I designed for a challenge 3 monster. We placed the 14 (from the standard 15, 14, 13, 12, 10, 8 ability score array) in Dexterity where it gains a +2 from the halfling race, making it a 16. That +3 ability modifier means its armor class is 13, right where a challenge 3 monster should be. Sure it has only 3 levels of warlock, but that few hit points is going to put it squarely in CR 1/8 territory. So we added as many hit dice as necessary to get it in the 101-115 range. Now considering that the skeleton gave it a damage vulnerability to bludgeoning weapons and an immunity to poison, I felt letting it have 10 hit points more than the normal value was wall justified. The attack bonus on eldritch blast is +4 but its 8 damage falls short so we added repelling blast, making it push its opponents away. This will make coordinating attacks more difficult for the players. It does not make up for it fully, but it is a consideration. Having said that, the halfling warlock skeleton casts burning hands as a 2nd-level spell. That will deal an average of 14 damage each time it is used, counting as 28 considering it damages more than 1 creature. That is close enough to the damage expected for a monster of that level and can be considered balanced for its level, as described by the DMG.
Halfling Warlock Skeleton
Small undead, chaotic evil
Armor Class 13
Hit Points 126 (28d6 + 28)
Speed 25 ft.
STR 8 (–1) DEX 16 (+3) CON 13 (+1)
INT 10 (+0) WIS 12 (+1) CHA 16 (+3)
Skills Deception +5, Intimidation +5,
Saving Throws Wis +3, Cha +5
Damage Vulnerabilities bludgeoning
Damage Immunities poison
Condition Immunities exhaustion poisoned
Senses darkvision 60 ft., passive Perception 11
Languages Abyssal, Common, Halfling
Challenge 3 (700 XP)
Brave. The halfling warlock skeleton has advantage on saving throws against being frightened.
Dark One’s Blessing. The halfling warlock skeleton gains 6 temporary hit points when it reduces a hostile creature to 0 hit points.
Eldritch Invocations. The halfling warlock skeleton has the following invocaitons: agonizing blast, repelling blast
Halfling Nimbleness. The halfling warlock skeleton can move through the space of any creature that is size Medium or larger.
Lucky. The halfling warlock skeleton can reroll a 1 rolled on an attack roll, ability check, or saving throw, but must use the new roll.
Naturally Stealthy. The halfling warlock skeleton can hide behind a creature that is Medium or larger.
Spellcasting. The halfling warlock skeleton is a 3rd-level spellcaster. Its spellcasting ability is Charisma (spell save DC 13, +5 to hit with spell attacks). The halfling warlock skeleton knows the following warlock spells:
Cantrips (at will): eldritch blast, mage hand
2nd level (2 slots): blindness/deafness, burning hands, charm person, command, comprehend language, hold person, protection from evil and good, scorching ray
Eldritch Blast. Ranged Spell Attack: +4 to hit, range 120 ft., one creature. Hit: 8 (1d10 + 3) force damage. A creature hit by the halfling warlock skeleton’s eldritch blast is moved 10 feet away from the halfling warlock skeleton in a straight line.