The idea of shadowbolt is to do some damage and set up an enemy for a whole world of hurt if they have some spell resistance.  It can only be used every other turn, which limits its usefulness,  I'd say that it's a fair trade-off considering that the Death magic doesn't give you any way to heal your troops, Heal being a Life spell with no cooldown and there being no way to stop it apart from the usual 'shoot the healer first' method.

Wildling Shamans typically have the Shadowbolt spell and considering the ease with which you can dispatch them when you have archers, it's fair that they get to throw a spell at you which isn't resistable and sets up Blindness.

It does like half as much damage as flame dart too. I'd say it's good design.

Making a spell which gives you a way to deal with unit with high spell resistance, and then making it not work on those same targets would be a mistake (imo).  If shadowbolt was to good I would lower the damage or the amount of spell resistance it lowers, I wouldn't make it resistable (and I dont think it need to be nerfed).

The debuff of Spell Resist does wear off fairly quickly, depending of course on how quickly the other player gets their turns (I think).  Typically when I do play Empire, I need to get my debuff or damage spell in there next turn or the Shadowbolt is wasted.

It's useless to be honest....

I've never felt that shadowbolt was broken, and the reduced resist and unresistable gives it some flavor so it's not just a "Death" Firebolt.

That's what happens when a damage spell is resisted. Most of the time damage spells has conditions where if it got resisted it still does half damage instead of nothing. Something that can easily be used to reduce the effectiveness of one part of shadowbolt (maybe if you resist you don't lose mres, or maybe you still lose it, but take less damage). Of course I don't think it's that bad at all, so I'm fine with it being left alone.

If you want to see an OP spell, you should really check out sunder. A good hero/sovereign will probably 2 hit most of the wildland bosses with that spell. It only works on elementals... but there's so many to use it on.