What might cause sudden double vision and vision deterioration?

Many possibilites: Several things can cause the problems you describe. Sudden blurred vision and sudden double vision both warrant immediate evaluation by an eye doctor. See one as soon as possible.

Many things : Sudden onset diplopia and vision loss can be a sign of increased intracranial pressure, intracranial mass, intracranial bleeding, neurodegeneration, vitamin toxicity, drug toxicity, infection, inflammation, trauma, and many more things. It calls for an immediate assessment from your doctor and ophthalmologist.