The Lucida Shard is an accessory that increases luck by 5. It can also be used as an item to remove confusion effects. One can be found in a treasure chest in Diabolic Hall.

How do I get into Sniflheim?

Sail to Sniflheim and dock there. The front gate is frozen shut so go around the back along the west side and unlock the magic door.

Where can I find serpent skin in Dragon Quest 11?

Dragon Quest XI Serpent Soul

You can get a lot of Serpent Soul from either dragons or demon monsters. The majority of them can be found in the Trial of the Luminary, Snaerfelt, or the Celestial Sands.