I didn't actually see the eclipse (wrong continent) but it definitely got the gears in my head turning. I wish I had art skills to draw the idea bouncing around in my noggin, but consider; corruption takes over Nightmare and Dream. They merge together into one entity called Total Eclipse.
Its magic is ice white, its body is pitch black. It retains Nightmare's 'texture' but it has Dream's ethereal glow. It's large... the intimidating stature of two Gods in one form, with an aura of unspeakable intensity. Despite its two halves famously being at constant war with one another, the entity is stable, unnervingly calm. It constantly looks faintly amused.
When it speaks, it speaks in either Dream or Nightmare's voice. If it's talking to someone who knew one of the two, it uses the voice they're most familiar with. It occasionally uses both at once - seemingly for dramatic effect.
It can trigger any number of feelings in someone with only a glance. It looks like it takes joy in doing so. Standing too close to it can cause uncontrollable tears.
It responds to the name 'Eclipse'. It has a delayed response to 'Dream' - and doesn't seem to react at all to 'Nightmare'.