Astral Corruption is a Global Enchantment that is available at Blood Magic 7, requiring 66 and 188 to cast. It has a chance to send a Horror to attack all those casting ritual spells or performing forging that does not solely use blood slaves. This chance increases with the number of gems spent on the action, respecting effects such as Forge Bonuses that alter the number of gems consumed1). It is functionally a lesser form of the Cataclysm.
The disruption this can cause to most nations occasionally causes this spell to be banned in some multiplayer games. It can also have a severe impact on diplomacy and international relations.
The presence of Astral Corruption sets the global horror level to 10 if it was less than that. For a full discussion of this and its effects, see the Horror article. Notably, this means that all eligible heroes have a 0.2% per turn to be attacked by the Hunter of Heroes, even if not horror marked. Similarly, all non-mindless Pretenders have a 0.5% chance to be attacked by the Eater of Gods.
However, assuming that the Cataclysm is not active the chances of a non-Mindless mage being attacked when forging an item or casting a ritual spell with a non-blood component is approximately given by:
((Mage's Horror Marks * 20) + 1 - Province's Drain scale + number of gems spent) / 35
Performing these activities with a mage with any number of horror marks is therefore extremely likely to result in the mage becoming attacked. For forged items with two different gem costs, the two components are evaluated separately. This seems to still affect ritual spells cast for free through magic items, despite them costing no gems.
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