The principle of opening a gate for a demonic army is the same as summoning a demon. You need to accumulate some critical mass of synergistic energy and open a rift between the worlds. The needed ingredients consist of some of the negative energy that is similar to the essence of the demonic realm.
The first part of the summoning ritual opens only a small rift to the demonic realm. Through this rift demonic energy enters into our world infiltrating into the ingredients and filling them with more power. With stronger ingredients, the ritual opens a larger rift this time, letting more demonic energy in to strengthen the ingredients. This ritual is repeated over and over again until the ingredients are strong enough to open a portal through which the demons can pass.
This ritual can also be initiated from the demon side. Sightings of Orshabaal and similar entities clearly show all characteristics common for such rituals.
Once the gate is open, demonic minions appear and boost the negative energy of the place by their sheer presence. Later, the demonic master is following. His appearance causes a drop of the negative energy as the demon lord is feeding on it to sustain his existence in our realm.
As long as there are enough minions to boost the negative energy of a place, demons can exist for a very long time in our realm. So in theory, it would also be possible that the gate between the two realms is kept open, allowing more and more demons to enter our world that can easily form a demonic army causing mayhem and havoc.
This book's content is an excerpt from Archdemons (Book).