The concept of the "deadliest" sword in history is subjective and depends on various factors, including the specific criteria and context being considered. While swords can be lethal weapons, their effectiveness in combat is influenced by factors such as the skill of the wielder, the design of the sword, and the historical context in which it was used. Here are a few swords that have gained a reputation for their deadly capabilities:

1. Damascus Steel Swords: Swords made from Damascus steel, a type of steel produced in the Middle East from around 300 to 1700 CE, were known for their exceptional sharpness, durability, and cutting ability. The layered and folded construction of Damascus steel blades contributed to their reputation as deadly weapons.
2. Katana: The katana, a traditional Japanese sword, is often associated with lethality due to its sharpness and cutting ability. Crafted using a combination of hard and soft steel, the katana's curved blade design and differential hardening technique allowed for devastating slashes and precise thrusts.
3. Khopesh: The khopesh is an ancient Egyptian sword with a curved blade that originated around 2500 BCE. Its unique shape and weight distribution made it effective for slashing and hooking attacks, potentially causing severe injuries.
4. Zweihander: The Zweihander, a two-handed sword used in Europe during the late medieval and Renaissance periods, was known for its immense size and weight. It could deliver powerful strikes capable of causing significant damage to opponents.
5. Tizona and Colada: Tizona and Colada are the legendary swords associated with El Cid, a medieval Spanish knight. These swords are often depicted as formidable weapons capable of delivering deadly blows.

It is important to note that the lethality of a sword is not solely dependent on the weapon itself but also on the skill, technique, and strategy of the wielder. The effectiveness of a sword in combat is influenced by numerous factors, and the perception of the "deadliest" sword can vary depending on personal opinions, historical context, and the specific criteria used to evaluate lethality.