Uilos, I've just noticed: in your second post you say "Shadow's Death" (= one shadow, so it's Mors Umbrae), but in your first post, you say Death of Shadows. So, if it's more than one, the correct translation is:
Mors Umbrarum = Death of (the) Shadows, or
Nex Umbrarum = (violent) Death of (the) Shadows, or
Umbrarum Nex = Shadows' (violent) Death
(word order is a stylistic choice)
Pluto, God of the Roman Underworld, was called: Umbrarum Rex (King of Shadows/dead spirits), so Umbrarum Nex would be a bit of a world play.