I’m getting a tattoo of the line from the Aeneid “The descent into hell is easy” and wasn’t sure if it is “fascilis descensus averno” or “fascilis descensus averni”. Which one is it? What’s the difference?
Anyway, both forms make sense. I would read descensus Averno as "descent through the entrance of the underworld" and descensus Averni as "descent of the entrance of the underworld". The word Avernus actually means literally "birdless" and refers to a place considered to be an entrance to the underworld. It does not mean "hell" directly, but by metonymy it makes sense to say that descensus Averni/Averno is "descent to hell".
I would recommend facilis descensus Averno. As I said, the difference to Averni is insignificant, but I wouldn't use that before finding it in an edition of the Aeneid. [As noted in the comments, there are editions with Averni, but Averno seems to be preferred. Both are valid choices.]