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Am I to understand it simply adds 15 to the wild surge roll? Because "positive effect" depends on the situation. And even then, wild surges don't exclusively get better the higher the roll. But I guess it's simply +15 nonetheless?
While we're at it, what's with the asterisk next to (Improved) Chaos Shield when cast?
For the people who don't have it but want a go at the topic: It grants "+15% probability that a wild surge will have positive effect"