Mathematical help with encounter difficulty

[Pinjata]

I'm setting up a final encounter for my short campaign and I'd like a bit of an additional assesment of encounter difficulty for my last battle.

PCs will be entering a small rural town and after they do a long rest, town will be at night (lots of houses burning, etc.) attacked by a large mercenary group. Attackers will be humans humanoids (Veterans from 5e, etc).

Players will be lvl 4. Thus we have four lvl 4 PCs, supported by three CR 7 NPCs. (PCs: Ranger, Cleric, Wizard, Sorcerer, NPCs: Fighter, Cleric, Rogue) Plus maybe a few (up to 6 commoners using Shortbows mainly)

With this setup, I'm thinking, I can toss at them 8 Hard groups (enabling for them to take 2 short rests, wheen they feel like) appropriate for a group of 7 lvl 5 PCs) Also, there is a chance of a Young Green Dragon strafing them with his breath here and there (dragon is cruising above the village durkng the assault)

This is a "rule of the thumb" calculation I am working with.

Should I change this encounter in any way and if so, how?

thanks :)

[EggKookoo]

One question I have is, are those three CR 7 NPCs actual NPCs or built as PCs? That may skew things pretty wildly.

[Pinjata]

One question I have is, are those three CR 7 NPCs actual NPCs or built as PCs? That may skew things pretty wildly.
They are:
- Buffed up Gladiator
- Assassin
- Lvl 5 Cleric with Cure Wounds only spells

My CR assessment was approximate.

[JackPhoenix]

I'd be wary about the NPCs. With 3 of them, all much more powerful than characters, it may looks like the PCs are playing second fiddle to them.

[Pinjata]

I'd be wary about the NPCs. With 3 of them, all much more powerful than characters, it may looks like the PCs are playing second fiddle to them.

Good point. They were already doing this once and they had great fun. They loved to place someone between them and the enemies to soak up the damage. :D

[Yakk]

Your CR assessment is probably wrong. L 5 clerics are usually not CR 7; PCs tend to have lower CR than level.

3 CR 7 NPCs next to a non-insane optimization L 4x4 party will make the party into support.

Using encounter building level math:
L4 PC is EBL 1.5
L5 cleric is EBL 2
I assume assassin/etc is similar. (L 1 is.5, L2 is .75, L3 is 1, L4-14 is 0.5 each)

CR 1/8 is EBL 1/6, CR 1/4 is EBL 1/3, CR 1/2 monster is EBL 2/3, CR 1 is EBL 1, CR 2 is EBL 1.5, CR 3 is EBL 2.
CR 4-14ish is EBL (CR-1).

Add up each side. If PC side has EBL 2x other side, is medium encounter.

Monsters 1/3 PC: easy. 1/2 PC: medium. 2/3 PC: hard. 5/6 PC: deadly.

(This is linearization of DMG guidelines for XP and monster number; it takes into account the monster number multipliers without having to use them)

So 4 L4 PCs: 6. 3 L 5 NPCs: 6. 12 commoners with shortbows (CR 1/8): 2.

Total: 14.

Veterans are CR 3, so EBL 2. 3.5 of them is a medium fight, 6 is deadly, 3 is easy. The dragon is EBL 7, also a medium fight, with a veteran hard, with 2.5 deadly.

...

I would use Guards for the NPC townsfolk, Orcs for the attacking grunts, Veterans for their commanders.

Townsguards: 1/6 EBL (Guard stat block)
Grunts: 2/3 EBL (Orc stat block)
Officers: 2 EBL (Veteran stat block)
Dragon: 7 EBL (Adult green dragon)
NPC allies: 2 EBL (as above)
PCs: 1.5 EBL (as PCs)

Encounter 1: Officer, 6 grunts (4 EBL) vs PCs+allies (10 EBL) - easy
Encounter 2: 2 Officers, 9 Grunts (10 EBL) vs PC+allies+12 townsguards (12 EBL) -- deadly

short rest (church, civilians). Civilians flee through escape tunnel during rest (very small tunnel cramped). Ask PCs to guard the church as they retreat. Works.

Then:

Encounter 3: Dragon strafe's church. Bunch of townsguards and civilians die. Dragon flies away, but with defences damaged 2 officers + 12 grunts (12 EBL) attack the church.

PC+Allies+6 allies+6 townsguards (11 EBL) have to buy time as remaining civilians flee. Players running away is plaubile here, as is losing allies.

Assuming they flee down the tunnel & close it behind them, they arrive at the safe location. The civilians are hiding there and recovering. The refugees flee.

The PCs get a short rest if they go with the refugees. Then:

Encounter 4. 2 officers, 10 grunts attack at the same time as a Knight with 2x HP (EBL 3) lands on the Dragon (EBL 7). The townsguard face down the mercenaries, as the PCs have to defeat the dragon + knight.

At the end of each round, roll for 1d4 Grunts and 1d6 Townsguard to die. (On a 4, 3 grunts and an officer; otherwise officers are last to die) plus 1d8 civilians (who are taking up arms - rocks, bows, whatever - in despiration). If 15 Townsguard die, the mercenaries slaughter 2d10 civilians next round then join the fight with the PCs. If the dragon dies, the mercenaries flee in disarray (roll 2d4 for how many die during retreat from townsguard).

The EBL math is based off XP is roughly HP*Dam*Accuracy*Defence, and encounter scaling is roughly XP^(2/3) for each monster added up linearly. This removes the "encounter size multiplier" fuzzy math making adding up mixes of monsters easier.

50 civilians will have fled down the tunnel even if the PCs don't help.
Another 100 make it if the PCs guard the church in the short rest.
Each round the PCs hold the church another 1d10 make it, to a max of 50.

Then subtract however many are killed by the mercenaries while the PCs fight the Dragon.

If the PCs don't fight the Dragon, 2d100 of the otherwise surviving church refugee civilians die (possibly all of them), as do all but 1d10-1d6 townsguard (min 0).

1d20 civilians survive elsewhere, plus 1d20 if the PCs guarded the church (distraction), 1d20 if they hold out until the church is empty, and 2d20 if they kill the Dragon.

Keep track of how many townsguard live at the end of each encounter. That is how many are left if the PCs win. (some of them are found elsewhere; it isn't those exact guards, as many of them are reused in later encounters).