The concept of the "deadliest" sword is subjective and depends on various factors such as the skill of the wielder, the intended use, and the specific historical context. However, there are several swords that have been renowned for their effectiveness in combat and their potential for lethal damage. Here are a few examples:

1. Katana: The katana, a traditional Japanese sword, is often regarded as a deadly weapon due to its sharpness, cutting ability, and versatility in combat. It was designed for quick and precise strikes, and its curved blade allowed for effective slashing and thrusting techniques.
2. Scimitar: The scimitar is a curved sword commonly associated with Middle Eastern cultures. Its design and weight distribution make it effective for powerful slashing attacks. The scimitar's curved blade concentrates the force of the blow, making it capable of inflicting severe damage.
3. Rapier: The rapier, a slender and agile thrusting sword popular in Europe during the Renaissance period, was known for its precision and ability to target vital areas. It was designed for dueling and was particularly effective in skilled hands.
4. 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 an effective weapon for slashing and hooking attacks, potentially causing severe injuries.
5. Ulfberht Sword: The Ulfberht sword was a type of Viking sword known for its high-quality craftsmanship and advanced technology. These swords were made of superior materials and were capable of inflicting lethal wounds in battle.

It's important to note that the effectiveness of a sword depends not only on its design and characteristics but also on the skill and training of the wielder. The lethality of a sword is ultimately determined by the combination of the weapon's capabilities and the expertise of the person wielding it.