I like the idea of having to catch lightning. Thats pretty cool. I would really like to see more of the ecology of this steel. Does it exist naturally, if it does: what is it like there? Are there any myths or rumors about it, or famous people who used it to do something great (even if untrue)? What is some cool local folklore about it? Is it hard to mine or purify, and what special precautions have to be taken by what group of people. Does it affect local trade anywhere. Can it be used in any alchemical process. Frankensteins monster?
Little bits of detail like that really take a solid idea, and make it a must have idea.
The evil shark people link is broken, but I like this idea, but I just happen to like the idea of whales as being more powerful and a threat to humanity, rather than it's victim. I can see the black leviathans coexisting in varying states of peace and war with things like Cheka's krakens.
It is simple and to the point. The information is given sparsely to tune it to ones own group or campaign with ease. I rather like it. I found myself thinking of Hellgate and how this is almost a like minded prison/ forgotten realm of similar torture. I wished for more information but realized it was very bare bones and didn't really need much more regarding information I couldn't fill out myself.
As long as yarnuk expeditions to the surface are rare, and the only members of their race encountered are the defective cast offs, people are probably going to do with them what they already do with lobsters. I don't see this being a problem, since defective or not, if there are more than enough of them in an area, they are going to breed, and there aren't many ocean creatures in shallow water that are going to be able to kill a one ton lobster with basic intelligence.
I can imagine the horror when a deep snailship surfaces near a coastal village, and the yarnuk explorers discover that the exiles have been captured, and are being bred, and worst of all, they are being eaten. Their boiled shells are polished into jewelry, and the undesirable innards are dumped into the water for scavengers to consume. Juveniles feeding on the discarded viscera of their own kin? Cannibalism, and murder?