Combat rating 22
1 Pit fiend (CR 20)
3 Fire giants (CR 9)
Combat rating 23
1 Pit fiend (CR 20)
4-5 Fire elementals (CR 5)
2-3 Efreeti (CR 11)
Combat rating 25
1 Pit fiend (CR 20)
3 Iron golems (CR 16)
Combat rating 27
1 Pit fiend (CR 20)
1 Ancient red dragon (CR 24)
2 Nycaloths (CR 9)
1 Ultroloth (CR 13)
How to Use – Combat Encounter (difficulty 25)
Stage 1: When the pit fiend sees the PCs, I would have a number of other monsters be around. The pit fiend gestures to the other monsters to attack, and then climbs up to a high spot, where it can watch the fight like a Ceaser at a Roman circus.
On the turn it goes up, it will cast Wall of Fire behind it, to prevent the players from going after it. You might also give it a bodyguard or two, to further disincentivize the players from attacking it. I would suggest an ice devil, or perhaps two Erinyes. The ice devil set up its wall just behind the wall of fire, making an incredibly difficult obstacle to have to get through. The Erinyes can use their ropes to drag any PC that goes past the wall of fire right back out.
If they attack the pit devil anyhow, possibly with spells or ranged attacks, have the pit devil point out to them that if they fight it now, when the lesser devils are still there, they’ll have a much harder time winning than they will if the wait until the pit fiend is ready. If they persist, then have the pit fiend join in. If they want an impossible battle, let them have one.
Unfortunately for your players, the pit fiend won’t exactly be a passive watcher. He’ll use his turn to toss fireballs at your players, just for the fun of it. I would suggest a small number of monsters, probably 3-4. [All the monsters in the minions/allies tab are at least resistant to fire, several are immune. All devils are immune.]
The difficulty level of this fight will need careful tuning. Too strong, and they won’t be able to fight the pit fiend afterwards. Too easy, and they’ll win before the pit fiend has the chance to throw almost any fireballs.
[I set up this stage of the fight because unlimited fireballs is a really cool power that the pit fiend never gets to utilize normally. The pit fiend’s melee attacks are so powerful that finding a situation where Fireball is the right option is all but impossible. This will hopefully solve that.]
Stage 2: Once the pit fiend sees his minions defeated, he’ll enter the battle. You can have his guards from before join him, or not, as you see fit. It’s not unreasonable to have the pit fiend decide to fight by himself, just so that the PCs will see his might. By the time he realizes that he’s losing, he’s already committed, and he’ll prefer to risk it all than to have to admit that he’s overwhelmed.
As the players finish off the first stage, have him cast a Wall of Fire into the middle of the battle, dividing the battlefield in two. The pit fiend will use its turn to walk over to the PCs, melee attack them, and then walk through the wall of fire to where they can’t attack it without going through and taking a lot of fire damage. If they do go through, then it can end its next turn on the other side of the wall of fire.
[Use the bite attack on a PC not currently suffering from the ongoing damage, as the damage doesn’t stack. The pit fiend probably has the wisdom to know who has CON saving throw proficiency.]
While this strategy will cause the pit fiend to take a few opportunity attacks, it’s worth it. Either the players will lose several attacks for each attack of opportunity, or they’ll take an amount of damage much greater than the attacks of opportunity when they walk through the wall of fire. Either way, the pit fiend wins.
This strategy will also prevent PCs from being affected by its fear aura, but the fact that once a PC makes a save, he’s immune for the rest of the day, means that most PCs won’t be affected for more than one or two rounds. As such, the pit fiend has no reason to be in a hurry to use it. [It will be used in the last stage, for all PCs not already immune.]
Stage 3: Once the pit fiend is down to less than half its HP, it will use its last Wall of Fire to put a circle around as many PCs as possible, prioritizing the spell casters. It walks or flies into their midst that turn, if it wasn’t already there. While they can retreat, they’re probably low on HP, and won’t want to take the damage of both an opportunity attack and walking through a Wall of Fire.
This stage is a tight fight, with the spellcasters not having the ability to retreat, and with both sides striving to make the other run out of HP first. This is where the pit fiend’s fear aura will be especially relevant, if it hasn’t been already.
If you’d like to drag out the climax further, and if this isn’t meant to be the end of your campaign, then when the pit fiend sees that it is going to lose it tells them about an extremely time-sensitive task. This can be a magical explosion spell that is going to go off, a hostage that has been left in an extremely precarious situation, or the like. The pit fiend won’t bargain, and it definitely won’t beg, but it will take advantage of a situation like this to prevent them from being able to finish the battle.
Since it’s leering at them, it can put itself into a defensive stance, so long as it’s not too obvious what it’s doing. It won’t wedge itself into a narrow, easily defendable area, but it should have no problem taking the dodge action to be able to turn aside their attacks with ease. Alternatively, disengaging or dodging and flying to a high area from which it can leer down at them.
If you do use this strategy, I suggest a simple obstacle on the way [examples: A staircase or hallway on fire, a door welded shut, a minor devil holding a defensive position] followed by them having to destroy the spell or rescue the hostage despite a precarious floor, a flood or fire, or devils shooting at them. They’re probably pretty near death by that point, especially if the pit fiend wasn’t the first battle of the day, so I wouldn’t make it too difficult.
If you do this, the pit fiend will take advantage of the opportunity to get away. Since the players will want to defeat the pit fiend, I suggest having it be blocking the way to a future boss battle or climatic scene, possibly the first one. The pit fiend will be blocking their way forward. You can either have it place both a physical barrier and/or its wall of flames in front of it, in which case it will throw fireballs down onto them until they draw close to it, or just start with melee.
The pit fiend can charge at them [it also wants closure for last time], possibly introducing itself with a Fireball; or it can defend a narrow passage such as a twisting staircase, forcing them to enter its fear aura if they want to fight it. You’ll want a sense of urgency, even if you don’t have an actual penalty for taking too long, and ideally, you’ll want to have a climax encounter afterwards. The two will combine to making the encounter deadly, and not a boring repeat of the previous encounter with the pit fiend.