The banshee can mask their form, they may only maintain that form as long as they do not take damage. The transformation cannot be very much bigger nor smaller than their natural form and they can only transform into humanoid/goblinoid races and are unable to use their Spells or other Boons while maintaining this form.
On tranformation they choose one singular form and this form cannot change or morph through magical means and can not be dispelled. The only way of forcing a Banshee to stay in their Natural form is by powerful magic (e.g.: Divine Word) or to watch them while they’re already in their natural form, as they cannot transform themselves while being seen