Once placed, the seal performs some kind of magical effect in the area it is facing. The exact area it affects varies from seal combination to seal combination, but is usually an equal sided cube placed in the direction the seal is facing.
The seal cannot perform magical effects all by itself. This is done by placing Runic Essences in them. See Seal Combinations for a complete list of all possible combinations.
It should be noted that while seals seem truly wondrous, they are not without a cost. When seals are operating, they cause a disturbance in the local Aura which can promote the creation of Taint. Using too many seals in a limited area can have disastrous consequences. Some seals are inherently more likely to cause Taint - the portal seals being a good example of this. Each time the portal is traversed, it tears a rift in the local aura which causes large amounts of taint to be released. You would be well advised to use portals only when absolutely necessary.
Crafting / InfusionEdit
Arcane Seals are created by infusing a gold ingot with Redstone Dust:
There are six types of Runic Essences: Magic, Air, Water, Earth, Fire and Dark. Each Essence corresponds to the type of crystal used when it was infused. Up to three Runic Essences can be placed on a seal and the combination of essences used determines what the seal can do. See Seal Combinations for a complete list of all possible combinations.
Once placed on an Arcane Seal, runic essences can only be recovered using a Wand of Reversal. Much like Thaumatic devices with upgrades, breaking the seal will destroy any and all essences on it and will only return the seal.
As a guideline of what Essences represent, see below:
The Crystal Ball allows you to see what a seal combination does when you use it on the seal. See Crystal Ball for the crafting recipe and more detailed information.