The Kunai is a harpoon-like weapon used most notably by Scorpion from Mortal Kombat onwards. According to Mortal Kombat Legacy, Scorpion was a master of the kunai. There are two kinds of Kunai: Big Kunai and Short Kunai.
The ninja spectre Scorpion chose this particular blade as his own favorite weapon in both life and death, as he continued to use it in battle and even slaughtering his foes and anyone who dared to get in his way throughout the entire Mortal Kombat legacy, especially when he shouts his famous catchphrase "Get over here!" as he pierces his opponents with this attack.
The weapon had evolved over the years; first with the Kunai attached to a rope, then with the Kunai attached to a metal chain. However, in the Mortal Kombat film continuity, including Mortal Kombat: Conquest, Scorpion's "spear" is a sentient, serpent-like being that emerges from a hole in his palm and strikes at his command.
The kunai itself has also appeared as another collectible Relic named "Scorpion's Spear" within the Konquest mode of Mortal Kombat: Armageddon. This was the most well-known title for the weapon until it was explicitly identified as a Kunai in Mortal Kombat: Deadly Alliance. In Mortal Kombat (2011), Skarlet has four short kunai on her right leg, which she can throw and attack her enemies with in kombat.
In Mortal Kombat X, Scorpion's apprentice, Takeda Takahashi, uses explosive kunai in battle. Along with this, Takeda uses serrated edge Whips with a kunai at their end to pull his opponents toward himself to inflict further damage, much like his master. Sub-Zero has ten short kunai on his left and right hips and arms. Shirai Ryu has four kunai on his left and right arms.
- In the original sprite-based Mortal Kombat games, the rope attached to Scorpion's Kunai would often appear behind Scorpion's hand or even behind him entirely when used, due to sprite and programming limitations of the time. The variation of length would depend on where the enemy had been hit or had blocked the attack. This issue persisted into Mortal Kombat 4, which while in 3D, behaved in a similar manner to the sprite-based games.
- In addition to his catchphrase, Scorpion's Kunai attack was always accompanied by the sound of an object cutting through the wind from Mortal Kombat II onward. This sound effect would be used up until Mortal Kombat vs DC Universe, which lacked a sound effect entirely, and effectively replaced with the sound of moving chains from the reboot onward.
- Reptile, Smoke, Noob Saibot, Chameleon and Triborg have all at one point been able to use Scorpion's Kunai, due to their origins as recolored ninjas using Scorpion's moveset (With the exception of Triborg). Smoke used a three-pronged variation of the attack after being turned into a cyborg during Mortal Kombat 3, although the attack was fundamentally the same; Triborg also uses it the same way as well, but only when he uses Smoke's Variation. Shujinko is also able to use the attack as a result of his training under Scorpion, including his own version of the trademark "Get over here!" catchphrase.
- In Mortal Kombat: The Journey Begins, Scorpion's Kunai was fired from an underarm-mounted mechanism built into his sleeve. Rather than spearing the opponent, it simply wrapped around them like a lasso in order for him to pull them towards him. In Mortal Kombat: Defenders of the Realm, Cyber Smoke's version of the attack was changed into a similar lasso-based attack.
- For many years, Mortal Kombat fans assumed Scorpion's Kunai and rope emerged from within his palm, due to the low definition of sprites and the widely-recognized depiction of the Mortal Kombat film, where his Kunai was in fact a living serpent that spawned from his palm. This depiction carried over into the film's sequel Mortal Kombat Annihilation and its spinoff TV show, Mortal Kombat Conquest, and was changed into a chainlinked serpent head for his depiction in Mortal Kombat: Defenders of the Realm. This was finally dispelled during Scorpion's appearance in Mortal Kombat: Deadly Alliance, which showed him actively throwing the Kunai-tied rope, and was further expanded on in Mortal Kombat Deception, where the Kunai is tied to his belt until being actively thrown at the enemy.