I've been waiting awhile to comment on this one because I've been trying to let it grow on me. The idea is interesting -- a bit munchkin-y, to be honest -- but I can see that it would work if you had good rules for blocking with shields in your game system.
What puts me off is really its construction. These are dwarves, right? For a weapon-returning shield I would have expected a more . . . mechanical solution. Maybe something that slingshots the missile around the dwarf and back at their foe. Maybe something that uses magnetic forces. The shamanistic rock substance didn't really do it for me.
Now, I can imagine a sort of "portable portal" variant of this that might make sense (though probably not for dwarves). Instead of a rock that actually sucks the spear out of someone's hands, the shield could be a two-way portal that will simply reflect the attack back out. The enemy would still be holding onto his end of the spear, but the pointy end would now be coming at his chest. No sucking force at all, and no reason the weapon couldn't be half-out, half-in.
Just my 2c. Even if you didn't change the construction or effects of this shield in any way, it could still use some expansion.
The idea is interesting but I have trouble visualizing it. Dossta's idea of a "portable portal" is a nice twist and is easier for me to see. It is especially nasty, since all of the attacker's force is twisted back around.
This sub leaves me curious about the Skkanden. They seem more "savage" than the average Dwarf. Are they more focused on stonework than metalwork? Is their culture more tribal/animistic in nature?
Seeing where the birth of its name came from it does not surprise me to think of it as a Chinese spirit of death. A death-bringer, but not in an evil and malicious sort of way. It is cold and callous for sure but not evil or nocuous. Wicked perhaps but it just doesn't seem like Horng Poh-Buh would strive to maliciously cause injury or pain. he is a bringer of death and death isn't pretty but he does say that no one shall take their own life as long as I am around. Kind of a Dr. Kevorkian with actual intent. I like it.
Although I'm finding it difficult to imagine exactly what it looks like. Perhaps that is also its intent. Like he is so horrible the minds eye won't allow you to actually look directly at him.
Thundershell Periwinkle -
A small sea snail that grows a sound-emitting shell. The shell acts like a minor Thunderstone. it makes very little noise, but it considered a novelty edible when eaten raw, as the slug inside the shell makes an appealing low roll of thunder as it is sucked from the shell.