Upon buying items from the banished sigil, you will start noticing effects happening on your screen... soon, it becomes scarier as you buy more items for it... up until you start seeing one of the effects that may get you killed unless you can dodge it...
Insanity effects are effects that are placed in there for gameplay balance reasons in order to prevent one user to become overpowered with rare items and not being punished for it.
The way insanity works is that there are many different levels/tiers you can be on that can cause different insanity effects. Upon buying items from the banished sigil, you will see a message "Your sanity is increasing...", stating that you will now encounter insanity effects on your screen. Continuing to buy more items from banished sigil and you will begin to see more and more insanity effects along the way. Up until you will see lethal insanity effects of which can either cause you to teleport to the z-gas, lasers to come up, or blades coming to your body to kill you. However, many of these can be countered and even preventable if you know how insanity works.
Insanity Lethal Effects
In order to prevent insanity, the best way is to not buy any item from the banished sigil. If you want the items from it, the solution would be to wait until somebody dies with it and then you can obtain the banished item without buying from the banished sigil. (NOTE: you do not obtain any insanity progress if you loot banished items and not buy them).
If you still want to buy banished sigil items but not to have insanity effects, then the solution is to not buy anything above 50 points. This means that you can still buy Hunter Chalice while not having any insanity progress. However, buying two of hunter chalices will get you to tier one insanity.