While going through this list of broken paragon paths we have found that, more often than not, the offenders were... well, focused on offense. Bonuses to attack and damage are the most common sources of unbalance in 4E, by a mile. On the other hand, if you have ever wondered what a broken defensive ability looks like, you need to look no further - Master of the Forge is your choice.
If you are optimizing a character’s defenses, every point counts, but you can certainly make some points worth way more than normal - particularly when you are close to unhittability from monsters of your level range. Master of the Forge is a paragon path for runepriests, which is effective in the hands of any multiclassing character, and allows you to make your allies’ AC almost impossible to hit by providing monstruous bonuses - as an at-will power. When most defenders happily take any +1 bonus to AC that comes over their way, and cry of joy when they find anything remotely resembling a continuous +2 bonus, something like Blessing of the Forge (a minor action utility granting a whopping +4 bonus to an ally adjacent to the runepriest) is a dream come true.
To see how this can become abusive, consider that a typical defender in plate and shield gets hit roughly 35% of the time against AC by enemies of its level. If the defender’s party includes a Master of the Forge, we can add an extra 5 points on top of that (4 from Blessing of the Forge, and another 1 from the extremely convenient Indomitable Steel feature), lowering this chance to a mere 10%, or a 19+ on the d20. This more than triples the defender’s survivability against attacks targeting AC, and makes attacking him an exercise in futility. Providing the bonus to non-defender characters is not as ridiculous, but it can still be quite strong as long as they have heavy armor.
I think is one case where simple number tweaking may do the job. I believe halving the bonus granted by Blessing of the Forge still leaves the paragon path at a very competitive power level:
Blessing of the Forge v.2 - Utility 12
At-Will - Divine
Minor Action - Melee touch
Target: One ally
Effect: Until the end of your next turn, the target gains a +1 power bonus to AC, or a +2 power bonus if the target is wearing heavy armor.
Special: You can use this power only once per round.
A secondary fix
With that out of the way, we can consider giving a slight boost to another feature of this path which suffers from the opposite problem: an excessively weak effect. Runes of the Blade Smith provides a continuous damage bonus, which is usually a great thing, but it amounts for a mere 1 extra point of damage, which seems way out of place for a paragon path. I’d rather have a slightly higher bonus, and make it into a power bonus to prevent stacking.
Runes of the Blade Smith (11th level): When you take a short or an extended rest, you can touch one weapon that you or an ally carries. That weapon then gains a +2 power bonus to damage rolls until you grant this bonus to a different weapon