Chthonians and the Vach-Viraj Chant (spoilers for The Great Old Ones)

[alois8113]

I've been running a conversion of the scenario from The Great Old Ones, The Spawn. During this scenario the investigators have a chance to learn a warding chant against the Chthonians that turn out to be the driving force behind the plot. Unfortunately, the only description of this spell is that it functions as a version of the Vach-Viraj Chant. I have never found a version of this spell or anything named similar to it that has an effect that remotely fits this description. Has anyone run this scenario? Does anyone have any idea what the effect of this chant might be?

Two points to note about this:

1. This is not the Elder Sign. Elder Signs appear elsewhere in the scenario and are used as a ward around a location. This is implied to be capable of driving the creatures below the town away from their current lair.

2. I am aware of the Vach-Viraj Incantation from the current version of the Malleus Monstorum. Again, it doesn't seem to fit the description in the scenario (it takes a long time, requires a permanent sacrifice of POW, and doesn't have much of a chance of success). Moreover, it doesn't work on Chthonians.

Anyone capable of helping?