The Tecpatl is a weapon used by Kotal Kahn. It is an Aztec knife that is mostly used in Aztec human sacrificing rituals that involves pulling out the human heart.
The Tecpatl was a 10 inch knife. Its handle is made of bone while the blade of the weapon is made of Obsidian stone, which is known to be sharper than steel because of its finer grain structure. Traditionally, it was used for human sacrifices by the Aztecs. But, it can be used as a side arm during a battle. It was also the short range weapon of the Aztec jaguar warriors. Although it has limited use on the battlefield, the Tecpatl's sharp edges would have been an effective sidearm. 
In Mortal Kombat X, it is one of the weapons used by Kotal Kahn. The Tecpatl was used in Kotal's intro when he slices his palm with the Tecpatl and blood rushes down. The other use of the Tecpatl was seen in Kotal Kahn's X-Ray move and his Fatality. 
- Kotal Kahn's Tecpatl was different from ordinary Tecpatls. Kotal's Tecpatl looks serrated and it is longer than normal Tecpatls.