List of Artifacts
Picking it up triggers a trap which floods the caverns. The mask seems to have 3 eye sockets on one side, and some lenses made of crystals on the other side.
Before the event of the game, Venus was asked by a Roach to protect the Ancient Tablet. After Team Snakemouth makes through her trial, she handles it to the team. It appears to be a piece of rock or wood with ancient languages written on it.
During Chapter 7, the Ancient Tablet is used to disable the shield that protects the sapling. During the final battle, it is also used to generate bubble shields, and always takes 3 hits to knock away.
Ancient Key Pieces
They are used to unlock the shell containing the Everlasting Sapling.
During the final battle, both key pieces can fire laser beams at their targets, or assemble together as a lightsaber to slash at their opponents for very high damage.
While not directly related to the main story, according to a Roach in the Giant's Lair, this crown also holds ancient power same as other ancient artifacts, and is stolen by the Wasp King. It is said that it amplifies any existing magic within the wearer. The Roach guard says that they tossed the crown out out.
If you buy all the Trinkets from the merchant in Metal Island, he will tell you he fished up a crown in the past and that he gave it to a young boy. That crown is heavily implied to be the Ancient Crown that the Roaches tossed, and the boy who wanted it was Hoaxe.