In the chapter where you play as the explorer at some point you'll find energy barriers. When all 3 are dispelled around this area a purple wall will drop. Remember where this place is.

Further along you will gain a summon trapper spell. Go back to where that purple wall was and summon a trapper. Take it through the hole and get it to stand on the circle tile. The rune will appear outside by you. Press A to kill the trapper and go get the rune. Now kill the horror that bursts through the wall and go through and kill the next horror with a gun. Now go over the bridge and use the brush to find the codex.

This rune allows you to align any spell except the summon spells to mantorok. The reveal invisible spell will actually turn you invisible and the dispell spell will dispell any magickal barrier that can be dispelled.