A sudden wild magic - Diana Wynne Jones

For aeons the mages of Arth, a neighbouring universe, have been looting Earth of ideas, innovations and technologies, all the while manipulating events and creating devastating catastrophes for their own edification.
Now this brazen piracy is threatening Earth with total extinction.
It is up to the Ring, a secret society of witches and warlocks dedicated to the continuance and well-being of mankind, to fight the virtuous, unbendingly traditional stronghold of Arth with an arsenal of psychological sabotage, internal dissension -- and kamikaze sex . . .