Undrave
Legend
Am I right to assume that the active property of the Cloud Rune (Skye, the one that deflects an attack to another target) takes effect without a save -- i.e. there is no save involved in the use of this Rune? That seems quite good to me too. Especially since it seems (again correct me if I am wrong) it can target a melee attack, but transfer it like a ranged attack (30 feet away). So the dragon bites the party wizard, but the RK negates that damage and transfers it to one of the dragon's orc minions, killing him with no save and leaving the wizard untouched?
It transfers the attack roll to a different target, that means if the target has good AC they can avoid the damage. Not sure why you'd want to add another failiure point here?
I've played a rune knight myself - it's pretty good but not any crazier than other good fighter or barbarian subclasses.
I could see slowing down the number of total rune activations (you still know 2-5, but can only use 1-4?), but I'd be more likely to keep that as a back-pocket option if the character is too good in actual play.
You could level lock Uvar, Skye, and maybe Ild (those are the higher-tier giants anyways), but I'd leave the rest. I could see why you'd want to restrict Haug, but I wouldn't want to make the weakest giant's rune hard to get. If you're sure resistance is too much, you could switch it to a bunch of temp hp (2 or 3 per fighter level?) which would be a lot less powerful but still feel pretty cool and thematic.
Temp HP is probably the best idea. I think the resist is strong at low level, but when magic damage start flying around it's not gonna do much. Temp HP remains good, especially if it scales with Fighter level, at all levels of play.