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Or did the nightsisters just made him that way?

The Nightpeople, for lack of a better word, are all force sensitive. All of them being descended from the rouge Jedi Allya.

Aside from that, Ventress and Talzin both sensed his strength in the force prior to his transformation.

Being descended from a Jedi doesn't necessarily mean one will be Force Sensitive. Theron Shan and Mother Talzin are excellent examples of this.

I always took what Talzin said to mean that she didn't harness the force through natural means.

Regardless, Ventress and Talzin did feel the power of the force in him.

This all prooves that nightsisters black magic is stronger then force powers.

Originally posted by NewGuy01
Being descended from a Jedi doesn't necessarily mean one will be Force Sensitive.

Yes, but even so, some populations tend to have a weirdly high number of force sensitives.

Why didn't the empire exterminate them during the jedi purge?

Originally posted by PTforthewin
Why didn't the empire exterminate them during the jedi purge?
The Empire didn't try to kill all force sensitives. It tried to exterminate all Jedi, that's why it's called the Jedi Purge, but it often took advantage of Force Adepts for use in the Emperor's employ.

Originally posted by FreshestSlice
The Empire didn't try to kill all force sensitives. It tried to exterminate all Jedi, that's why it's called the Jedi Purge, but it often took advantage of Force Adepts for use in the Emperor's employ. oh I see

I speculated on this in a former post. Savage Oppress was force-sensitive, but much less so than Maul; that being the reason Maul was given to Sidious and not him. His natural force powers were augmented by Nightsister magicks. Talzin is not Force sensitive instead using learnt magicks to accomplish feats on par or perhaps even higher than Force sensitives. The ritual she was attempting with the Bardottans was her attempt to connect with the Force, and then become truly unstoppable.

Yes he matched Ventress in hand to hand.

Obviously he was.