Just one thing: the rodian never meet malak or revan, however he knew exar kun personally, he even said he had already look at him. Also, he lived 40 years under yavin 4 influence, so it's quite natural he thinks exar kun is the most feared sith ever. However, i don't remember kun killing an entire planet like taris, whith billions of people. This is prettyy much evil...
Anyway, this is pointless since he was no force sensitive (we can even use force persuade on him if we want and he was not able to resist). Also, when he saw revan for the first time he was a lightside user so he had no reason to fear him. Anyway, Master Vrook had to know Kun's power since he was a master jedi by that time, yet i showed you that statement in which he says that "The force flows through you like no other student we had ever seen before", thus making revan>exar kun in force power since kun was once a jedi padawan himself...
I think the sith'ari was Vader, due to him the sith became stronger then ever (this time the actually conquered ALL the galaxy), yet he destroyed the sith once and for all (canonly speaking of course).