There are three situations, one is a complete hit and the other is a complete injury. This is easy to understand, and the harm of this painful curse is fully accepted; The second is spelling mistakes. It is completely harmless; The third is partial resistance, and some are partial resistance every jump.
3. partial resistance, the target is higher than you or he has shadow resistance. Well, this is magic defense.
4. Shadow Arrow gains 86% spell damage bonus. So your 1000 dark damage is1000 * 86%+500-1000 * 86%+501.
Here is also considered according to the situation of 2. Partial resistance causes less damage, as can be seen from the combat record.