D&D 5E I need a big bad for a campaign

[generic]

My new campaign begins in a town that is troubled by the formation of a cult known as The Society Of Blood-soaked Midnight. The cult is secretly led by an evil green hag. I have an adventure that will entertain my players for about three levels, one problem: I have no idea were to go with my story when the mysterious Society has been destroyed, I want a continuing story, help!

 

[Phion]

Would you want to continue that theme or monty python it with a "now for something completly different"?

 

[Tormyr]

1. What level are they now?
2. What level will they be when they finish off the hag?
3. How long do you want the campaign to go?

 

[DeanP]

The hag is part of a coven of hags. The coven of hags serve an unseelie Archfey.

 

[Phion]

Maybe they get a reputation after sorting problem and sent to deal with another troublesome spell caster?

 

[Slit518]

The Big Bad Evil that is in control of the whole operation is an Ithilid/Beholder hybrid! This particular Ithilid can have it's head detach from the body, leaving it an empty husk while waiting for the head to return!

This Big Bad Evil also has an army of Umber Hulks waiting to do it's bidding in a hive-mind fashion, which are all linked to the Beholder brain.

The Beholder portion of the creature can also spawn Gauths out of it's giant eye, spawning 1 Gauth a day! It is looking for a way to speed this process up.

 

[Ancalagon]

the green hag was sacrificing children (or whatever evil thing) in an attempt to keep an evil stronger evil thing at bay. Now the imprisoned dragon/vampire/fiend etc is free!

 

[Storyteller Hero]

The Hag has a second-in-command, a human child with sorcerous potential that she raised into adulthood. The second commander is away from headquarters at the time that the Society collapses, and swears vengeance against the PCs for killing "Mother" upon hearing of the news.

 

[Eltab]

The hag used to belong to a coven. The rest of the coven still survives and is active. The coven members decide to plunder her lair and maybe try to rob the PCs for whatever they plundered off her.
The coven might send some muscle to 'keep the PCs busy' when the coven checks out the old lair. This will alert the PCs that the story is not yet ended.

 

[Ath'kethin]

Absolutely go with the "hag is part of a covey," angle, but have the other two hags polymorph into cats or something. Your players might suspect something is up with the old lady and her pets, but probably won't even come close to guessing the truth.

It's bending the rules a little, but as a DM that's kind of your duty.