In the middle ages the Eastern Roman Empire and the Bulgarian Empire were in constant wars for hundreds of years. In the late 10th to early 11th century Roman Emperor Basil II is trying to add Bulgaria to the Eastern Roman Empire. The Bulgarian Tsar Samuel is doing everything he can to prevent this. They have several battles but the war still continues.
However in 1014 AD they meet at a place called Kleidion.

A historian named John Skylitzes gives this brutal account of the battle:

"Emperor Basil II continued to invade Bulgaria every year without interruption, laying waste everything that came to hand. Samuel could do nothing in open country nor could he oppose the emperor in formal battle. He was shattered on all fronts and his own forces were declining, so he decided to close the way into Bulgaria with ditches and fences. He knew that the emperor was always in the habit of coming by way of what is called Kiava Longos and Kleidion, so he determined to block this pass and thus prevent the emperor from entering. He constructed a very wide fortification, stationed an adequate guard there and waited for the emperor who duly arrived and attempted to force a way in. But the guards stoutly resisted, killing the assailants and wounding them by hurling [weapons] from up above. The emperor had already abandoned the attempt to pass when Nikephoros Xiphias, then commander of Philippoupolis, made an agreement with the emperor that he would stay there and make repeated attacks on the enemy’s line while Xiphias would (according to his own words) go and see if he could do anything profitable and likely to solve their problem. He led his men back the way they had come. Then, trekking around the very high mountain which lies to the south of Kleidion and which is called Valasitza, passing by goat-paths and through trackless wastes, on 29 July, twelfth year of the indiction, he suddenly appeared above the Bulgars and came down on their backs with great cries and thundering tread. Completely taken aback by the unexpected nature of this attack, the Bulgars turned and fled. The emperor dismantled the abandoned defence-work and gave chase; many fell and even more were taken prisoner. Samuel was only just able to escape from danger, by the cooperation of his own son who stoutly resisted those who attacked, got his father onto a horse and led him to the fortress called Prilapon. They say that the emperor blinded the prisoners, about 15,000 in number, with orders that one man for each hundred be left one eye so he could be their guide, then sent them back to Samuel. When he saw them arriving in such numbers and the state they were in, lacked the moral fortitude to endure the shock; fainting and darkness came upon him and he fell to the ground. By applying water and perfumes to get him breathing again, his attendants succeeded in bringing him back to himself some- what. As he revived, he called for cold water to drink. He got it, drank it and then suffered a heart attack; two days later he died on 6 October."

Source [web.archive.org]