Monadic devas are angels—by definition neutral good. For once, though, that doesn’t mean it’s hard to come up with encounter ideas for them.
First off, as guardians of the Ethereal and Elemental Planes, they are likely to haunt the very portals plane-hopping PCs seek to use, and they may try to test PCs’ intentions or bar them outright from passage. (I’m not alone in this reading; Ultimate Magic refers to them as “stubborn” and prone to “demand[ing] outrageous gifts of servitude.”) Their habit of lending out their magic +3 morningstars can put them at odds with those who don’t want to return the gifts, or who use them in ways the devas find objectionable. And when in doubt, you can always fall back on the fallen angel trope, swapping out the cure/good/holy attributes for their evil opposites.
A party is unknowingly infected with xill eggs. A monadic deva will stop at nothing to keep them from accidentally infecting their home world with the loathsome outsiders. Worse yet, the eggs have begun to corrupt the party’s auras, causing them to detect as evil.
Adventurers find a magical +3 morningstar, only to have its monadic deva owner come looking for it a few months later—and he wastes little time resorting to force if the adventurers resist. Should the matter be peacefully resolved, the angel recruits the adventurers to find out what happened to the weapon’s last owner, a cleric of St. Rhysgard who sought to proselytize to the syrinx (see the Inner Sea Bestiary). Once that mystery is solved, the deva could also use some help driving a kyton and his Advanced belker allies out of a noble djinni’s court on the Plane of Air.
One sect of monadic devas eschews the customary mace, favoring flails and scourges that they use upon themselves. This order of flagellants wanders the Ethereal Plane in squads, believing their suffering draws the attention of night hags, xills, and other threats aware from the Material Plane. They are quick to turn these same flails against others, however—and not just evil creatures. They kindly but firmly require those who come to them for aid (particularly for the removal of curses, disease, fear, and fell magic) to join them in their abasements—and only heal them after suffering has expiated the supplicants’ sins.
—Pathfinder Bestiary 2 27
In campaigns featuring servants of the Elemental Evils (as in: comma, Temple of), monadic devas can be great allies.
Also, did I really just base an adventure hook off the dictionary definition of “monadic,” which refers to a protozoan?
Yes. Yes, I did.