Too overpowered, let′s take a scenario into mind, you see a enemy running at you, all you have to do is cast a substained destruction spell that costs the same substained mana as you regenerate. Easy win, on any mob, just 1 spell and run around and hide.
And if we get spells to lock doors again you could also just cast a substained spell, go through a doorway and close it + lock it with spell, then you can kill any dragon or monster in 1 spell and you can just sit there yawning waiting for it to die.
And I can go on for a thousand more scenarions for tens of different spells and so on and so on, way too overpowered.
Edit: I say this all considering that I take it that there would not be line of sight limits or anything like that.
And if we get spells to lock doors again you could also just cast a substained spell, go through a doorway and close it + lock it with spell, then you can kill any dragon or monster in 1 spell and you can just sit there yawning waiting for it to die.
And I can go on for a thousand more scenarions for tens of different spells and so on and so on, way too overpowered.
Edit: I say this all considering that I take it that there would not be line of sight limits or anything like that.
Line of sight rule would make sense as well. Or even limiting it to cast on self spells.
Other than that, your scenarios can already be done by summoning a beast and then hiding.