The term "deadliest" when referring to Japanese swords is subjective and depends on various factors such as the skill of the wielder, the purpose of the sword, and the specific historical context. However, the katana is often regarded as one of the most deadly and effective swords in Japanese history due to its design, craftsmanship, and the martial arts techniques developed for its use.

The katana's design and construction allowed for a combination of factors that contributed to its effectiveness in combat:

1. Sharpness: The katana's cutting edge was honed to an exceptional sharpness, allowing it to deliver devastating cuts and slashes.
2. Durability: The katana's construction, with its combination of hard cutting edge and flexible spine, gave it the ability to withstand impact and retain its sharpness during combat.
3. Balance: The katana's well-balanced design facilitated precise and controlled movements, making it effective in both offense and defense.
4. Cutting Ability: The curved shape of the katana, along with its sharp cutting edge, enabled it to deliver powerful cuts with minimal effort, maximizing its cutting ability.
5. Versatility: The katana was versatile in combat, allowing for a range of techniques, including slashes, thrusts, and parries.

It's important to note that the effectiveness of a sword ultimately depends on the skill, training, and mindset of the wielder. The lethal nature of any sword, including the katana, lies in the hands of the person who wields it.