Custom monster abilities?

[Ras_Nsi]

I'm trying to create a custom creature as there's no DM content for Fall of Plaguestone, but I can't seem to just add a supernatuaral ability to a creature. There's a wolf with a two action acidic line attack with a basic reflex save to defend against it.

However, you cannot seem to create custom attacks like this, only a ranged or melee attack.

Am I missing something