ASN-121 was a repurposed ASN courier droid, modified to function as an assassin droid and utilized by the Clawdite bounty hunter, Zam Wesell. In the time leading up to the Clone Wars, specifically before the start, this droid was deployed in a failed assassination attempt targeting Naboo Senator Padmé Amidala. Its mission was thwarted, and Zam ultimately destroyed it when Jedi Knight Obi-Wan Kenobi sought to use it to find her.
During the height of the Separatist Crisis which took place in 22 BBY, Mandalorian bounty hunter Jango Fett and his associate, Zam Wesell, were hired by Confederate Head of State Count Dooku to assassinate Senator Padmé Amidala. Dooku's goal was to influence Trade Federation Viceroy Nute Gunray, Amidala's political opponent, to join his cause. After an unsuccessful attempt to kill her by bombing her starship on Coruscant, Fett rendezvoused with Wesell at night, providing her with a container holding two venomous kouhuns for a more subtle approach to eliminating their target.
Zam placed the tube containing the kouhuns into ASN-121's delivery system, after which it immediately flew to execute its objective. Upon arriving at Amidala's apartment where she was sleeping, the droid used a laser to create a small, round hole in the window to release the kouhuns. Despite the presence of Amidala's astromech, R2-D2, the creatures almost succeeded in killing the senator until Jedi Padawan Anakin Skywalker sensed them and swiftly eliminated them. Following the deaths of the kouhuns, ASN-121 attempted to escape, but Jedi Knight Obi-Wan Kenobi tackled it, launching himself through the window.
With Kenobi holding on, the droid desperately returned to Wesell, failing to shake off the Jedi by slamming him into a building wall along the way. As it approached the bounty hunter's location, Zam used the sophisticated scope of her KiSteer 1284 projectile rifle to target ASN-121 for several seconds before finally locking on and destroying it with a single shot, sending Kenobi falling through the Coruscanti traffic.
Soon after ASN-121's destruction, Zam, its owner and destroyer, was killed by Jango to prevent her from revealing his identity to Kenobi and Skywalker.