User blog comment:FTWinchester/Darkness as the overall Big Bad/@comment-9249716-20160126223945/@comment-5619706-20160127182012

A determining characteristic of a Big Bad is that it is the one who creates the master plan, and not necessarily the one who enforces the plan--that's where "the Dragon" comes in. The Dragon acts as the muscle (i.e., Caleb to the First Evil, Alastair to Lilith).

And the First Evil actually did pretty good in manipulating the characters especially in Conversations With Dead People, and in manipulating Robin Wood against Spike. My only actual qualm with the First Evil is that it was defeated in part by Wolfram & Hart due to the amulet Angel gave Spike.

The First Evil wasn't the only malevolent force--it just happened the be the one who started sin itself. Remember how Illyria said the Wolf the Ram and the Hart were barely stronger than the vampire in primordial times? What nakes the buffyverse interesting is that there are several key players--something Supernatural could have had if they capitalized more on other powerful deities, fairies, Death, Godstiel, Eve and Leviathans.

This is why seasons 8-10 felt so unnecessary to me. Because after stowing away Purgatory we are back to heaven vs hell since our sources of evil were very limited.