I'd decided that the hostile force had done divinations before they set out for the party's town, and were confident no one in the village knew they were coming. (It was a month-long trip out to the hinterlands, and the party learned about their approach only after the group had set out.) I figured each day the Pontiff would cast divination to see if anything would threaten him personally in the next 24 hours, and that he used a fair number of spells casting sending to coordinate his conspiracy back in more settled lands.
The plan was fairly simple. The item they needed to bind the demon was near this town, and it was dangerous for a person to handle it because they might go made. So the Pontiff brought enough people to cow a small town. They'd march in, take control, and then the Pontiff and his specialists (four 7th level wizards, an 11th level bard, an 11th level transmuter, and a 9th level fighter; plus he's a 15th level cleric) would go get the item and perform the ritual. The PCs were an unanticipated snag.
The day before the ambush, I ruled the Pontiff got the warning that an invisible talon would strike from above (the party had already dubbed their attack 'operation bloodtalon'), so he bound an erinyes to keep watch. He did a follow-up divination about how large the hostile force was, and learned that it was about 20 foes.
I decided that the beast's howdah was enchanted to act as an arrow catcher, so any ranged attacks against the Pontiff would not be a threat. His AC was high, he had as good saves as I could manage, something like 80 HP, and had cast death ward on himself. He figured that whatever tried to get him, he could survive, and then the army would take it out. I considered a variety of defensive spells, but almost nothing in 5e lasts more than a few minutes, so the spellcasters would have to just wait for the attack and then counter-attack.
The army marched like this (each group was typically 4 abreast):
- = 10 ft. gap
A = 10 arcane trickster archers
E = 10 eldritch knight pikemen
S = supply cart, ox, 2 pikemen
I = illusionist w/8 pikemen
C = conjurer w/8 pikemen
T = transmuter & bard w/8 pikemen
V = evoker w/8 pikemen
P = Pontiff atop huge beast, with erinyes and 2 pikemen
N = enchanter w/8 pikemen
A = 10 more archers
H = horse scouts
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(rear) A - E - S - E - S - E - S - E - S - I - C - T - V - P - N - A - H (front)
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Basically it was a column about 300 feet long. They were expecting an attack at some point during the day, and had scouts out who were able to cast silent image to light up the sky if they spotted anything. The pixies were clutch for the party.
Surprise Round: Once the column got to the hill, the centaurs broke from cover and fired two shots each, and the warlock mezzed the groups marked N and A at the front of the column. Then they rode out of range of most spells, and back behind cover.
Round One: The army gets off the road and presses up against cover (retreating would have been less advantageous, since the ox-drawn carts of supplies would not have handled the confusion well; better to get to cover and count on superior firepower). The pontiff figures he has a solution against archers and readies blade barrier to stop the next volley. The erinyes fires off a few shots. An earth elemental gets summoned at the front of the column and attacks the riders.
At this point, the bard (invisible and with cover from brush) breaks cover and flies down to beside the huge beast. She figured someone might have see invisibility and wanted to use the monster's body as cover. I ruled that since the erinyes was distracted by shooting at the centaurs, it was possible to get from cover to cover without being spotted, and the bard rolled Stealth and beat the Erinyes's passive perception.
Round Two: The eldritch knight pikemen at V and T blast the elemental with magic missiles. The rest of the army is alerted that there are probably forces on the other side of the road, so they start marching through the brush. Then a sleet storm goes up to slow their approach.
The erinyes keeps shooting at the fleeing warlock, and takes out his centaur steed. That's when the PC bard snatches the pontiff and flies back and high enough that the party, who is on the far side of the hill with readied actions, takes him out in one round.
Round Three: The bard gets the Pontiff to the party, and they deliver a coup de grace, just in case. The party actually gets overconfident and tries to kill the erinyes, but then the army makes it to the edge of the sleet storm in the heavy brush. They know that next round the army will have a clear shot at them. Worse, some mages teleport in and mezz a few PCs.
Round Four: Luckily the caster PCs and NPCs (who all had dimension door prepped) teleport the group away.
Less than thirty seconds.