The Eldtritch Ward is a powerful sigil that is used to ward off the Great Old Ones. It has been used for thousands of years against the Old Ones by those whom oppose them, and so far have successfully driven them off and prevented their chaos from spreading across the lands and plunging the world into darkness.
As powerful as this warding is, it is also highly difficult and complex to fully master. This is likely due to the fact that it requires the blood of one who is untainted by the 'gaze' of an Old One, crushed bone marrow of a white dove, the stomach bile of a dog, and lastly it must be drawn upon a floor with the properties of Maplewood. These ingredients can be frustrating to locate, and ever since the imprisonment of the Old Ones, they barely have been on the market.
The Eldtritch Ward can also prevent the ilk of the Great Old Ones from causing harm to others. It is potent to the point where it can even harm their offspring to the brink of death due to them being much weaker than their parents. And it can even be strong enough to ward off the more powerful Outer Gods, albeit for a short amount of time. Beings like Nyarlathotep have even found the warding to be quite bothersome.