Ancient, pre-Imperial droids, the Mourners were pre-programmed with instructions to kill anyone on a list of Jabba's enemies who the Hutt viewed as possible instigator to an assassination, starting with the most recent. Some of the potential victims of the droids were Eugene Talmont, Bib Fortuna, Lady Valarian and the Rebel cell leader Jun Seros.
They intoned repeatedly the following chant: "We mourn for the exalted Jabba. We cry for the magnificent Jabba. We grieve for the illustrous Jabba."
Seros called in the help of visiting Rebel operatives to take out the droids, aided by stormtroopers, local police and some of Valarian's goons.