I completely wiped out every kuriouate city with HN. I led barbarians to their cities, then i killed their defenders, and the barbs took the city. My top competitor for land was taken out without a hit on my civ.
That's an interesting strategy.
I have not had as much success with taking out defenders with HN units (like the Stooges) and having the barbs come along to capture the cities. The problems:
1. The ai can churn out defenders one per turn somehow, so there is always a unit there to defend against the barbs.
2. Unless I have a small stack of HN units (like all 3 Stooges), the ai will really focus their attack on the HN units around.
3. I guess I don't know how to lead the barbarians. I have made a city defenseless and waiting many turns for a barb to show up. If the map is full of AI civs this, of course, may never happen.
4. The barb unit takes the city (they never raze unless in is 1 population), but then it is quickly taken back by the ai as that Goblin is not a strong defender.

So, you have to take out the ai civ defender again and so forth.
4. The biggest problem I have had is that the barb units will more often than not attack my HN unit (and die) rather than take a defenseless city. This is particularly true with barb units with more than one movement like Lizardmen, Wolf Riders, and Goblin Chariots.
It is a great strategy if it works though.
The Svartalfar world spell didn't make much sense to me in my one game I played them as I was constantly at war with other civs, so I never used it.