I am stuck here and I hate asking for help or ideas because sometimes it feels like I am asking for someone else to write the story for me, but this has stumped me for awhile now.
Basically my group of characters find an old ruined shrine. The shrine was dedicated to the demonic forces of old, and all shrines and temples of the demonic lords lay in ruins or are fully destroyed. But they find this one and make pacts with the dark forces. With their newfound powers, they plan to wreak havok upon the armies and lands of the Gods that they hate.
Here's the problem : I want to make them a serious threat to the many kingdoms in my world but I am not sure how to make them an actual threat. Yes they can use dark magic and all that good stuff but they are only a small group. They have no armies, no fortress, and plus they will be fighting other armies that are blessed by the Gods. I really don't want to be lazy and just say "100 years later, they are now a serious threat."
Perhaps I can have them summon a host of demonic spirits? I was also thinking they could go to villages or towns and take hostages and then force demons into the bodies and they would be possesed ina way, but I'm not sure.