Complex sentence: It turns out that when Grim told Billy to throw salt over his shoulder for good luck, and he threw pepper instead, a bad-luck snail crept into his brain, making sure that Billy has bad luck forever.
Split into simpler sentences: It turns out that when Grim told Billy to throw salt over his shoulder for good luck, he threw pepper instead. This caused a bad-luck snail to creep into his brain, making sure that Billy has bad luck forever.