Aware of the arrival of the USS Enterprise to Ardana in 2269, the Disrupters (malcontent Troglytes) began "despoiling" the cloud city of Stratos by, as in one instance, placing a mortae into a work of art located the council gallery. In explaining to Enterprise officers, James T. Kirk and Spock, why anyone would defile a work of art, the Ardanan high adviser, Plasus, explained that art meant nothing to the Disrupters, and that the mortae was "the only form" that they understood. A short time later, two sentinels discovered an non-permitted Troglyte attempting to leave the city. Plasus confronted the Troglyte with the mortae used to damage the artwork, asking him if it was his.
Until the arrival of the Enterprise, mortaes and thongs were the only weapons the Disrupters had to use to bargain with the Stratos city-dwellers. Vanna, one of the more intelligent Troglyte-Disruptors, nearly changed that when she temporarily acquired Kirk's Starfleet type 2 phaser, after having taken him hostage. This acquisition was short lived, however, because Kirk was able to quickly reacquire it from her.
After succumbing to the effects of zenite gas, Plasus challenged Kirk to hand-to-hand combat, while both were trapped in the caverns, when he asked Kirk if he was "as brave with mortae as [he was] with a phaser?" As Kirk threw his phaser aside, he stated "both will kill," before taking on Plasus's challenge. (TOS: "The Cloud Minders")
The mortae was designed by Matt Jefferies. Several sketches depicting early concepts of the tool appear in the Star Trek: The Original Series Sketchbook (pg. 91) which describe it as a "miners moss knife."