The Giant Mole is a members only boss which is combat level 110. He is a large, non-aggressive beast, but a dangerous one nonetheless.
When you engage him in combat he will fight until you have damaged him enough to get him worried. From this moment on, you will need to be on your toes, as he will burrow (and hence move around) extremely quickly. It is wise to take a team, and split up to combat him as he appears throughout the area.
As he has stretched and sleek skin, you will find him weaker to puncturing attacks, such as Stab or Ranged. On his death, he may drop mole skin or claws, which can be taken to Wyson as proof that he has been destroyed.