The Death Watch

From Fettipedia

The Death Watch was a ruthless group of mercenaries who had once been members of the Mandalorians but left because of their insatiable appetite for greed and war.

The group's leader Vizsla led his group in slaughtering the family of Jango Fett that had been aiding the Death Watch's sworn enemies led by Jaster Mereel. A betrayal within Mereel's own group by fellow member, Montross, would later lead to his death. However, his successor Jango would lead the True Mandalorians in decimating their enemies once and for all.

References

• Jango Fett: For All Seasons