Earth Dome is similar to the Earth Wall that Ticka proposed to use against the fire the other day in that it creates a construct of something between dirt and stone. The difference between them and the reason they are separate spells is the complexity in creating a dome that doesn't collapse in on itself immediately after construction.
That was why Nia needed to focus and someone to protect her from the unknown monster while she focused.
As Alec watched Nia focus on drawing the spell formation, he couldn't help but think that this was an awfully roundabout way of doing things just because they couldn't find the monster with their current abilities.
But it should be the safest option as long as his Simple Barriers could hold up.
Otherwise, they would have to scour the entire hall for the monster that could be the size of a finger or smaller.
