There is a enchanted entrance in the north-east of the Sharkman Camp. It is the only entrance of the tomb.
In the night of full moon, the Sharkman Prophet will use the Band of Prophet to open the entrance. Followed by a group of Sharkman Priests, the prophet will bring offerings to the Soul Stone of Ancient Beast.
You understand? The band of Prophet is the key to enter the tomb. It looks like we have to do some nasty things to the Sharkman Prophet.
Take down the Sharkman Prophet and take the Band of Prophet. Be careful though, based on my observation, there is the prophet is always followed by a ferocious Cyclops, who is very tough to deal with.