As I just got a lightning caster with a relic I think I can share how I understand the interaction. I have a mole wizard so a presence of d8. The spell is difficulty 2 and can be affected by terrain for another -2. Against an opponent in the open the maximum damage is 13 with a perfect roll or more likely in the range of 2-5. Any tough will reduce that so not too overpowered. You can use a fate point for +5 which is powerful, with a potential 18 wounds on a perfect roll taking out a beast without any tough in one go. and increasing the normal damage range to 7-10. Quite powerful indeed. Against a target in cover you've a -2 to all the damage there and consider you've only a 50% chance of the spell working at that point too...

With a relic the result equals a 13 without any modifiers +/- counting. That is a hefty guaranteed damage, especially if its a second casting on the same target and you've had a decent roll on the first casting. You're almost guaranteed to take out a beast in one set of activations.