Armus was a malevolent entity created by an unknown race. Armus was born as a by-product of a procedure in which a race of "titans" brought out from within themselves all evil and negative attributes that had bound them to destructiveness. … see full wiki
The episode in which Armus appears is one of the worst of TNG. The idea that all of evilness was pulled out of a race and embodied in one creature is ludicrous and the implementation as a glob of petroleum sludge is even worse. As was demonstrated in "The Enemy Within" it is the evil in us that when properly channeled and controlled, makes humans great. This is one of the few episodes of the series that I have never been able to watch a second time.