The Ten Dons is the nickname for the group of the 10 Mafia leaders of the 10 territories into which the six continents of the Hunter x Hunter world are divided. In reality they don't act by themselves but use their special bodyguards, the Shadow Beasts, a group of the best 10 fighters within the Mafia.
After a large number of Mafia members and auction clients in Yorknew City are massacred by the Phantom Troupe, the Ten Dons send the Shadow Beasts to deal with them, not knowing who they are. After the Shadow Beasts are killed subsequently, they put large bounties on the Troupe's heads and hire professional assassins, including Zeno and Silva Zoldyck, to eliminate the Troupe's members. Their plan nearly succeeds--the Troupe's leader Chrollo Lucilfer is almost killed--but fails at the last minute because in a countermove Chrollo previously hired other members of the Zoldyck Family Maha, Illumi and Kalluto to assassinate them, a mission that is accomplished. Because of their deaths, Zeno and Silva no longer considers Chrollo a target, spare his life, and leave Yorknew City.