I expanded a bunch when running
3e's Lord of the Iron Fortress in 3.5.
Part of it is set in the Gate Town of Rigus on the Plane of Concordant Opposition next to the gate to the Lawful (evil) plane of Acheron known for eternal warfare.
I expanded Rigus to be filled with Lawful Evil and LN warlike mortal(ish) factions such as hobgoblins, fire giants, duergar, devil tieflings, zenythri law-touched planetouched, asherake (from
Complete Minions) and others. It is ruled by a mercenary warlord council who sell use of their armies and is a place known where you can hire some big armies.
When the party showed up as part of a murder investigation whose clues had led there a force of about 200,000 hobgoblins were doing military exercises outside the gates. Partly as a show of force to the
half a million strong horde of orcs camped outside of the gates who had shown up shortly after the city's largest army (of goblins) had disappeared in a
cloud of magical birds.
So the party conducts their investigation with that as a backdrop and as they were finishing up their investigation and concluding they needed to go into the Gate, surprise! Two factions betray the city and the orc horde pours in to pillage and take over the city and access to the gate. The party was like 15th level I think and I got to have fun with lots of common orcs, bunches of low level barbarians, a huge raid leader they encountered who was 16th level and so on with chaos as buildings were burned, tons of fighting, and so on as they tried to make it to the gate.
In part I added this as a tie in to the history of LE orcs with Acheron and then current CE orc themes.
In AD&D orcs were LE and Gruumsh and Maglubiyet originally were both LE gods who eternally fought on the Hells but eventually then to Gehenna and then in
Manual of the Planes their eternal afterlife war with regenerating racial warrior spirits moved to the eternal battlefields of Acheron.
It was a lot of fun to play around with massive orc armies narratively and a bunch of 3e orc stat blocks of wildly different CRs I had which went well with the barbarian CE themes and the chaos of the battle.