The STH training droid is the most advanced combat droid ever created. It was built to Darth Taral's detailed specifications for the purpose of training his new Sith recruits. The STH can hold its own in multiple forms of combat with even the most seasoned organics and given enough time to learn and adapt, can even prove to be a challenge for a trained Sith Lord. The Imperial Order of the Sith employs 500 units as of 15 NE.
STH droids are used for a number of training purposes, but none so impressive as those outfitted for lightsaber training. These special units are programmed with hundreds of melee styles, including all known lightsaber-specific forms. These droids are the heartiest of the different models, skinned in lightsaber-resistant phrik armor plating and sporting a reinforced chassis capable of standing up to the extreme punishment a Sith Lord can dish out.
Their plating, being lightsaber-resistant, cannot be damaged. However, the droids have hundreds of sensors that register blows. These sensors allow the droids to simulate injuries by cutting power to whichever section would have otherwise been damaged or destroyed by the lightsaber blade. This gives the Sith the most realistic simulation of combat outside of another organic.
Because the droids' AI continuously learn and adapt from the Sith they face, over time they accumulate the collective skills of every Sith they have dueled against and become extremely deadly combatants. The most veteran droids are usually reserved only for Masters of the highest skill.