A massive rock smashing sound launched from a catapult or similar siege weapon. It could work as an asteroid crashing into the Earth as well. Whatever you want really, get creative!
For more music, sound effects and logo & ident files please visit my profile page at http://audiojungle.net/user/dac_audio
And for my AudioJungle exclusive tracks you can go here: http://audiojungle.net/user/crebbs
Thanks for listening!