Complex sentence: When the Romans finally caught up with the Carthaginian host the butchery began, which left Hasdrubal with only 6,000 men, who then fled onto a mountain top without any water supply.
Split into simpler sentences: When the Romans finally caught up with the Carthaginian host the butchery began. Hasdrubal was left with only 6,000 men, who then fled onto a mountain top without any water supply.