Still kind of prototyping
One thing that D&D5e introduced was the concept of Advantage. This allowed them to give a bonus without actually giving a bonus number. In Fate of the Norns, Advantage will mean "you may pick a second color that counts as successes on a wyrd". This is most applicable to Skill checks.
Still kind of prototyping
Mental damage as written allows for stun-locking. Most players hate that happening to them, and the Norn doesn't either. It removes agency and makes your turn lacking action. So, considering options on house rules
Option 1 - mental damage does not hit In-Hand runes. If the In-Hand pile is chosen, it is treated as physical damage. If you have runes in other piles (In-Play, Contingency), it acts as it does now.
Option 2 - Runes hit with mental damage don't move down into essence. Instead, they are turned over and turn into unbound colorless runes. They can't be used for Metas or Active Powers at all, but still can be used for Cinematic Actions which don't require a specific color and still allows for Morphing. Any that are already in place as Metas are popped free of the rune chain and move to essence during clean-up. Contingencies are still pretty messed up here, because presumably you declare an Active power on a contingency, and those are no longer bound to active powers. If you are generous, you can let the contingency continue with Cinematic Actions that make sense. Runes become colored and bound again once in Essence.
Right now, I like Option 2. It gives an interesting effect.
Really, you should just replace Size with Encumbrance. It starts making FAR more sense. It's still kind of weird that QR increases Encumbrance, but at least you aren't shrinking two handed swords and stuff.
When it says "Level +6" or similar, that is not your level +6. It starts at 0. So, Gate Black Skeleton gates a Level +6, meaning it gates in a level 6 skeleton. If you amp it, it gates in a level 12, etc. Nuance of the writing.
Most powers have multiple Sources. If you have something like Regenerating Attack (Perform an Attack action, Heal +1, and Recover +2 ), each of those become their own Source. You can affect each of them individually with your Metas. You might look at the power like this
Regenerating Attack [Amplify Multi Weapon]
Source: Perform an Attack Action
Source: Heal +1
Source: Recover +2
And you could affect your Attack with Weapon, your Heal and your Attack with Amplify and Multi, and your Recover with Multi only for and end result that looks like
Attack [Amplify Weapon]
Heal +1 [Amplify Multi]
Recover +2 [Multi]
This is not well called out in the book, but has been confirmed to be the case.
Still in prototyping
Cover is a condition that is somewhat similar to the Shroud condition. The Norn may rule that it is not possible if a line between the attacker and the defender is not broken.
If you are adjacent to an object or a creature that is one size level smaller than you
you may pay a P rune. For the round, anyone attacking you who's line of sight goes through the adjacent creature must pay 1/4 their destiny to attack you.
If you are adjacent to an object or a creature that is the same size as you
you may pay a rune. For the round, anyone attacking you who's line of sight goes through the adjacent creature must pay 1/4 their destiny to attack you.
If you are adjacent to an object or a creature that is larger than you, you may not be attacked by anyone who would attack through the adjacent creature
Change Death Charge to Trample from Denizens of the North on the Ulfhednar Active Board. Death Charge is weapon only, in the middle of all the "i'm a wolf" powers. The Warhorse gets this change, as per the author, thus it makes sense for the Ulfhednar.
Is this sane? well, it is a weak attack, so that +4 is really only a +2. This makes it in line with death charge - consider a size 4 unarmed guy is pulling 5/2 = 2DF attacks out of this. A Size 4 guy using Death Charge can get that with a standard 4 handed weapon ((4+1)/2).
But what about Blood Wolf? Blood wolf is a rune by itself, and the maintain is another rune. Compare at a per-rune level. Plenty of comparable stuff.
But the book! The author argued for it being there for an interesting build. I don't think that's quite enough of a reason.
Despite what the book says, this is not how it's going to work.
The {Interrupt} can be played during another action. It doesn't necessarily stop the action, but it can happen during it.
Essentially, the {Interrupt} type says "you can ONLY use this during another action". It doesn't matter for most of them, because most are Defends and you can't preemptively defend. But for a few examples it does matter.
Contingencies effectively allow you to give your power the type of {Interrupt} under a set of conditions. If that never comes up, you don't get it, it goes back to Essence at the end. A risk, but potentially rewarding. Thus, you can use Contingency to perform "Opportunity Attacks". You can also use Contingency to try and trip someone during a move action.
Essentially, Interrupts work how you think they would.
Not a house rule, but an explanation, initiative is drawn for once per combat, unless situation dictates a moving out of initiative. Thus, initiative manipulation powers (i.e. the stalo) are handy.
Taunt is pretty strong and agency removing. To help, two things will be introduced
Taunt is directly countered by Shroud. If you have 1 point of Taunt, and get 1 point of Shroud, the 1 point of Shroud counters the 1 point of Taunt entirely, and vice versa. This is somewhat similar to Charm and Possession, though you can't Charm Possessed people.
If you have a power that hits multiple targets, as long as it hits the taunter, it counts as "spending your runes". That means, explicitly, if you use the Multi meta and attack the taunter, you can hit multiple people. If you want to pile on Multi metas, that's fine too. Just makes sure you hit the taunter. Includes Area effects as well.
I expect there might be further modification to this and Shroud. I really don't like the "spend 1/4 of your runes". It makes a queer stair step on your power curve. For example, you have a wyrd of 4 and someone hides, you spend 1 rune. If you have a wyrd of 5, you spend 2 runes and are just as effective as someone with a wyrd 4. Why do you have a wyrd of 5? Why do you get punished there? I've not found a suitable alternative.