Chronology
During the series' first two seasons, Charmed episodes featured mostly-unrelated encounters with the isolated and unorganized individuals haphazardly representing the forces of Evil. The main villains were not clearly established on a recurring basis, nor was any shadowy, manipulative, serious backroom power introduced, in contrast to later seasons, which featured strategic villains who used numerous surrogates in making sustained efforts to attack the Charmed Ones. Consequently, the first two seasons showed villains who were not well-pronounced and did not play longer than a few episodes, as the writers hadn't yet settled on the latter as part of the successful serialized formula, which they employed in later seasons to feature the sisters fighting against several "Big Bads".
One consequence is, the first three years of the series, which very much involved Prue as the principal series heroine with Piper and Phoebe in definite secondary roles, have a different feel and style from the last five seasons, which involved the Halliwells' half-sister, Paige Matthews. (Paige was introduced when Prue (Shannen Doherty) elected to take other work and was written out of the series.) The latter seasons also feel and progress quite a bit differently because of the soap opera appeal; the writers employed serialization, which enabled them to present cliffhanger aspects, although without overuse of that technique.
Here is a list of the major villains in Charmed.
Read more about this topic: Major Villains In Charmed