The black clouds foreboding rain cleared. What a relief!
We were thus spared the onslaught the rain.
I think "We were thus spared the onslaught OF the rain."
But this whole passage is a little too stuffy for its meaning. It speaks about a personal response to a familiar event, but it uses relatively rare and formal words -- literary words, maybe -- to do so.
A person who is recounting this event would speak more casually, I think.
"The black clouds that spelled out rain cleared. What a relief! We were spared being caught in a downpour."