The specific use of "fall to" is not at all the same as "fall on". In fact I would read it to mean that Avalon lost (fell) to whoever "these lands" is.
Avalon rained down on X. Death and blood poured from the sky.
"Rained down on" is a clearer metaphor for suddenly and overwhelmingly entering a place and having a deadly war there. If you say only that blood (whether red or not) poured from the sky it might not be clear it's a metaphor since blood is a liquid you could literally pour. So adding another word makes this more clearly a metaphor.
I also think X should be much more specific than "these lands" unless perhaps your narrator has been established as telling the history of the current location. "The kingdom of Whoever." "My home." "This golden valley." "The once-hidden glory of Wherever."
Also, keep the tense consistent. If Avalon falls or rains, then the blood pours. If the blood poured, then Avalon fell or rained.