When people talk about the hordes of the abyss, they are talking about dretches, born in droves from evil souls and forced into slavery or to the front lines. Their low CR also means that even beginning parties can get a taste of demon fighting in their very first adventure.
Like most campaigns, this one starts in a tavern. A ragtag group of would-be adventures discover they all have something in common: a loved one who spontaneously turned into a dretch. They join forces to find answers and hopefully stop the awful manifestations.
A party is walking through a planar market when a mob of dretches tears through, having escaped from the slave auctions. Their vrock handlers do not care how many market-goers are mauled as long as the dretches are rounded up, and attack the party if their intervention harms their livestock.
Dretches are born of the sin of sloth, and likewise often serve spellcasters too lazy to reach for greater power. A local hedge wizard’s habit of summoning dretches is a sign of his slow descent into evil and his growing search for easy shortcuts to power.
—Pathfinder Bestiary 60