How and when were the demons trapped by the enemy

[666IsMyName]

There's something I don't understand about this and I'm reading it very often. How the enemy trapped the demons? Did they literally trapped them or they stopped them from contacting/helping us?
Why didn't other satanists before HP Pythia free them?

I find its very confusing, so can anyone shed more light on it?

HAIL SATAN
HAIL Baal Zebul
HAIL THE DEMONS OF DUAT