SunRazer
Back From The Dead
Other than helping Sion virtually kill the Exile with their Lightning, they're Blademasters, but we'll get to that later.
Here's something for the standard Triumvirate assassins:
The Sith students of Darth Revan and Darth Malak are many of the same Jedi Crusaders that once fought for the republic. Already proficient in the Jedi arts, these marauders acquire new nefarious talents studying at the feet of Headmaster Jorak Uln - one of Exar Kun's original Sith acolytes.During the Dark Wars, surviving Sith students submit to the triumvirate.
-- Knights of the Old Republic Campaign Guide
These Sith Lords in particular apparently have the Blademaster appearance, which means:
Established by Darth Revan and heavily utilized by Darth Sion, these surreptitious butchers specialize in practical modes of Jedi execution. They drain the Force from their targets to augment their own powers and expertly turn captives into Sith with a combination of torture, Sith poisons, and the dark-side-drenched masks they wear.The most elite of these assassins are the Bladeborn - Sith blademasters who sometimes use lightsabers but more often wield dreaded tremor swords. Secluded on Malachor V, the Sith Assassins become pivotal tools for the Sith Triumvirate.
-- Knights of the Old Republic Campaign Guide
Traced to the Bladeborn, a Sith offshoot dedicated to sword mastery, these cortosis-faced weapons were given to “masterblades” who survived no less than 10 lightsaber-wielding warriors in combat.-- Knights of the Old Republic II: The Sith Lords: Prima Official Game Guide
Remembering that the average Jedi of this era > the average Jedi of the PT era.
It also mentions in the first quote above that these Assassins are supposed to be secluded on Malachor V, which makes them the top Sith of their time:
Only by fighting your way through the Sith's strongest guardians can you earn the right to confront your nemesis.-- Knights of the Old Republic II: The Sith Lords: Prima Official Game Guide
To reach the structure's core, where Darth Sion and Kreia await, you must face a legion of elite Sith single-handedly. -- Knights of the Old Republic II: The Sith Lords: Prima Official Game Guide