Detect Evil on Acheron

[Voadam]

...but... it's not evil, though. There's arguably just as many evil creatures on the material plane as on Acheron. It's a Lawful plane, not an Evil one.

That's a tough argument.

Acheron is home to the spirits of orcs and goblins, rakshasa, bladelings, achaierai, rust dragons, axiomatic and fiendish things, and armies and accompanying things (scavengers, undead, etc.), as the major populations.

 

[cmrscorpio]

In 3.x, Acheron doesn't have an evil descriptor, it isn't described as "Mildly Evil-aligned", just "Mildly Law-aligned". However, the DMG does say that the River Styx flows through it, and it exclusively uses the "Hellish Encounters" table for random encounters (as opposed to Mechanus, which uses both Hellish and Celestial). The Manual of the Planes goes a step further and actually calls it a "Lower Plane".

In Planescape, Acheron was Lawful Neutral Evil.

So, yes, I would allow a paladin who cast detect evil while on the plane to pick up some background evil, but probably on the lower end of the spectrum.