Look for the chat bubble on the Column, an ancient voice will talk to you. Insert the Dragon Body Parts and you will be warped to the cave entrance which is the center of the map. If you choose the other option, you will eat the parts and lose 10% HP.
The entrance portal will only open for 30 seconds if you miss it you will have to bring the items and open the portal again.
For party usage, have a priest bring the items and then memo the cave entrance at the center of the map if possible. Warp a bunch of people to the center of the map, then have someone give the items to the Column to enable the portal so everyone can get in at once.
If you want to leave the center island, talk to the Column at the left side of the Entrance.