Designing the Boss
Here’s what I’ve found works best for creating melee final boss monsters in D&D
Hit Points –
This is the best place to start when designing D&D final bosses. You want the boss to be tough enough that if it goes last in initiative, it isn’t going to be dead before it gets to act. Look at how much damage each character can do in one round of combat. Assume the fighter, monk, bard, and druid (and druid’s companions) all hit and deal their max damage. Assume the wizard casts their most damaging spell and rolls maximum damage. If the Bard has Haste, assume everyone will get an extra attack. Add all that damage together and that is the minimum number of hit points your monster should have. Then, take the average level of the party and multiply it by 12. Use whichever number is higher.
Since this is primarily going to be a melee baddie, you want her to be able to reliably hit the PC with the best defenses. Take the highest AC of the PCs and subtract 5. This gives you a target number for your To-Hit bonus. The reason why you want your baddie to hit reliably because she will get fewer attacks each round that the PCs. This assumes that the PCs are fighting no one other than the demon. It gets trickier if there are multiple enemies that the PCs are fighting, because the more enemies that you have to keep track of, the more likely you will forget to use some cool attack or ability.
The amount of damage that your baddie should be doing in a melee attack should max out at the monster’s to-hit. Since you know what weapons the baddie is using, look at the damage die of the weapon with the best damage. A spear does d8 damage. Take To Hit -8, and that is its melee damage bonus
Number of Attacks and attack bonus per attack
You want your baddie to have more attacks in a round than any of your PCs. This probably means looking at how many attacks the monk has, then adding 1. Does the demon have a bite attack, wing buffet, tail attack, or some other appendage that it can use to attack?
Since your baddie is wielding multiple weapons (carried weapons or other forms of attack), each weapon gets an attack at the full To Hit. If you have any attacks left, then give an extra attack at To Hit -5 until you have cycled through the each weapon. If you still have any attacks left, calculate at To Hit -10.
AC should be something that your melee PCs can reliably hit. Take the to-hit of the weakest melee PC and add 13 to it.
Demons don’t have a favored saving throw. Pick one to be at To Hit -2, and the others at -3.
If the baddie needs to make a skill check for something it would logically be able to do, use To Hit
Special powers and attacks
If your baddie has any special qualities added to its attacks, you should drop the damage of that attack. For instance, if the baddie has poisoned weapons, a stun attack, or a fiery aura that damages those next to her.
Spells and spell-like abilities. Stick to things that buff the demon or that are area of effect against the PCs.
Do not give the demon healing powers…unless the PCs have a way to nullify them.
Make the baddie big so that it has reach and can melee attack from further away
Final touches for the stats
Demons have a whole set of innate resistances, vulnerabilities, and abilities that can be found on page 40 of the Monster Manual
Lead up to the final fight
D&D assumes 4 encounters in a day in which they must expend resources. Let them fight the demon as encounter #3, that way they should still have resources, but not their best ones.
Know how the PCs are going to learn about any weaknesses or tricks as to how the fight must be won. Have a backup plan of how you are going to convey this information.
Notes on Running the Final Fight
If they manage to kill the baddie in one round due to critical hits, good for them.
If you come to realize that my advice on how to build the baddie was grossly wrong and it is going to wipe out the party, give it fewer attacks per round and/or reduce the damage that it deals
Spread the love, and by love, I mean pain. Do not use all of her attacks against one PC, unless that is the only PC within range. Spread them out, but if one PC has to take more attacks than the others, then it should be the fighter.
Keep Moving. Have the baddie move every round. Even if it is just a 5ft step, use it. Move closer to PCs who use ranged attacks if possible. Don’t get trapped in a corner, unable to move.
Attack each PC at least once, in melee if possible.
Demons often have the ability to summon more demons. If you use this option, either use demons that are very simple to run (don’t have any special abilities to keep track of) or have the main demon retreat from the fight until the PCs have defeated the fodder.
Give the PCs something to throw/knock off the side of the ship. (see previous note)
Charge the Wizard. It is worth the Attacks of Opportunity just to see the look of terror on the wizard’s face. Summoned minions are great for this.
Some demons have the ability to Teleport, or Dimension Door (see Keep Moving).
If the PCs have been hitting the baddie for 5 rounds and they haven’t reduced the demon’s hit points to below half, start doubling all the damage that the demon takes.