This is it. This is where the Ancient will appear to lay our world to ruin. All I have to do is incant the correct banishment ritual with the invading god's rune, and the madness ends. - Examining the Banishment Ritual.
The Banishment ritual is a four-rune spell needed to banish the Invading god.
Obtaining the Banishment Ritual
To obtain the banishment ritual, the player must complete Dungeons, where they will receive either a hint about one of the three gods, or a piece of the ritual. The ritual itself consists of four runes, three of which are fixed. The last remaining rune will be the rune that denotes the god you are trying to banish.
The player does not necessarily need all four pieces, as you may have found the three fixed runes. In this case, as long as the player knows what god they want to banish and what rune belongs to that god, they may safely cast the ritual.
The banishment ritual never uses a given rune in two fixed positions. For instance, if Eu appears as one of the fixed runes, it will not appear in either of the other two (though it may still repeat as the god's rune).
Casting the Banishment Ritual
Once you are ready to cast the Ritual of Banishment, head to Stonehenge.
Once you are there, enter the dungeon there and you will be prompted to find the underground ritual chamber. This chamber can be accessed through a semi-circle shaped opening with ladder in it near the floor. The entrance to the chamber is always behind a locked door.
Once you are in the chamber, cast the Ritual of Banishment as you would any other spell. If any of the fixed runes were incorrect, you will suffer a heavy sanity penalty. Should you banish the incorrect god, this results in Ending D (Mistaken Identity) and a game over.
If you cast the spell correctly, you will be transported to a realm where you must fight the Ancient. Try to enter the fight with as much Sanity and as many bullets as you can manage, as this will help make the fight easier.
Depending on your Sanity level, you will get either Ending A (Victory) or Ending B (Victory - Insane). If the Ancient kills you or if you get too close to its invading body, you instead get Ending C (Ultimate Sacrifice).