Great miracles are world-changing miracles that you can do occasionally as an archon that will go down in legend. Instead of providing set mechanics for these, we recommend you discuss between sessions with the GM the miracle you want to do and how it will affect the world. The cost of a miracle is generally calculated as follows:

5 + size cost + effect cost = total connection cost

Size is how big of an area or how many people the great miracle deals with and follows this scale:

  1. Affects an individual
  2. Affects a family line
  3. Affects a town
  4. Affects a country
  5. Affects a continent
  6. Affects the world

Effect is how world-shaping the miracle is and follows this scale:

  1. Trivial
  2. Small
  3. Moderate
  4. Potent
  5. Intense
  6. Cataclysmic

The point of a great miracle isn't to give your champion a new gift or grant your city an improvement to a development. In fact, players aren't even allowed to suggest mechanics for how a miracle would play out unless the GM specifically asks. The point isn't to provide a mechanical edge but to make roleplaying more interesting. 

Some examples. 

Destroying a City: This is a cataclysmic event on a city-wide scale so it costs 5 connection to start with plus 3 for the size plus 6 for the effect for a total of 14. This is cataclysmic because it completely reverses what the area is like and has a major effect on story. 

Creating a New Moon: This is a potent event on a worldwide scale so it costs 5 connection to start with plus 6 for the size plus 4 for the effect for a total of 15. It is potent because it changes the tides and demonstrates your power as an archon, plus has other astrological effects which would change how prophecies are determined.

Creating a New Monster in the World: If you start off with a single breeding pair, it's probably a moderate event on a family line scale, so it costs 5 connection to start with plus 2 for the size plus 3 for the effect for a total of 10. We assume a moderate event if it is a decent change in the animal's nature. Smaller changes like making pink rabbits would be one connection less in cost, while a flying, fire-breathing, colossus of a rabbit might be an intense event, or even cataclysmic if it completely changes how the nearby ecosystem functions. 

Every time you gain five levels you get a free great miracle which you can use. Using it reduces the cost of a great miracle by 10 connection. Great miracles take intense planning, preparation, and work to pull off. You cannot send them down on the fly, getting some champions out of trouble or smashing an enemy army in one swift disaster.

To counter the effects of a great miracle, you must pay the same connection cost that the original archon. Unless your archon can perform the overrule archon miracle, you cannot directly cancel out another archon's great miracle. You may, however, create an effect that undoes most of the harm. For example, if an archon causes your city to be perpetually under a rain of fire, you can make its citizens immune to fire. If an archon causes someone to be horrendously mutated into some hideous form, you can surround that individual in in illusory glamor so the hideousness is undone.