I never understood why they wasted the heretics so quickly. Thus far they've been the most powerful thing on in TVD/TO universe more so than even the originals. I guess the writers wrote themselves into a corner with them and had to find some contrived Rayna the vampire slayer ex machina, and she wasn't even stronger than them. It's just been hard to take these big bad sirens seriously when one of the prior big bads is a werewolf/vampire hybrid and also an original...and another who can shphion all of that away.
this show has always had a problem with escalation. they hype up their villains - making them uber-evil AND uber-powerful, but the problem with that is that an evil, powerful villains HAS to be taken down by the heroes at the first attempt otherwise retribution will be swift and final. So they either:
1, make the villain stupid and fail (Pearl, Anna and the tomb vampires) 2, they make the heroes suddenly stronger in a way that defies their own rules (Jeremy+Elena killing Kol or Damon being able to manhandle Katherine etc) 3, they woobify the villain so he stops being a villain (Klaus - who killed Jenna and held a 500-yr-old grudge on Katherine because she refused to roll over and die for him - suddenly starts drawing ponies for Caroline & has a Mean Daddy), etc 4, they create another Bigger, Badder Villain to take down the Big Bad. Like in this example, Rayna for the heretics (and the Armoury for Rayna, and the Sirens for the Armoury and the Devil himself for the Sirens).