Assault of stone, a true classic. Everyone would steer clear of the hills for miles. It was hilarious watching people deploy their armies to avoid it. It was almost more effective as a threat than as an actual spell. The 5th ed plague spell was possibly the nastiest piece of magical shennanigans I ever saw, and funny to boot. I also really enjoy gobbling down warpstone to really power up those spells...I still remember with dread the old high elf spell that could move hills, not the sort of thing you'd want to face when you have loads of artillery.