The Family of the Blade were a guild of assassins present within Romulan society.
Its members were known to be well trained and capable of being planted into organizations where they could masquerade as harmless individuals. This was an act as when given a mission, the professional assassin was capable of shedding their disguise and taking out their target once the order was given.
In order to ensure they were not captured, they made use of a philotosan chip hidden in their mouths, which were triggered to unleash a quick acting poison that killed the assassin to prevent any interrogation.
Sharibor Krel was an agent of the Family of the Blade. (VOY - Dark Matters novel: Ghost Dance)