'rather suddenly' is a correct and usable phrase in written English.
You can use it to describe a sudden or unexpected event that occurred. For example, "The meeting ended rather suddenly when there was an emergency."
It has all happened rather suddenly.
"And then, rather suddenly, the marriage turned into a nightmare".
They found that it all happened rather suddenly.
"This program was started rather suddenly," the receptionist explained.
Now, rather suddenly, Mars is showing new signs of life.
His career in terrorism rather suddenly went up in smoke.
But the stromatolites disappeared rather suddenly, and we don't quite know why.
Air America started to play rerun programming rather suddenly Thursday evening.
But, he added, "rather suddenly, the marriage turned into a nightmare".
Long abandoned, the underground cities of Cappadocia have rather suddenly been rediscovered: by the produce industry.
Then, rather suddenly, he'd purchased a new car and opened his own business, selling men's clothing.
