Yor global enchantment was overpowered..????

[Psitticine]

There can only be 5 global enchantments active at one time. When you (or somebody else) tries to cast a 6th one, one of the existing ones is selected at random and its power is compared to the power of the new spell (i.e. the number of gems used in their casting are compared, which is why it can pay off to put extra gems into a spell). If the new spell is stronger than the old, the old one is dispelled and replaced by the new one. If the old one is stronger, the new one fails and you get the message about it being overpowered by the existing global enchantment.