The king lathas Amulet is a reward from the Biohazard quest. It has no stat bonuses, although when it is received, the chat screen displays King Lathas gives you a magic amulet.
The amulet was originally used as a means of alerting the player to return to King Lathas to start the next quest in the Plague City series, the Underground Pass. The stone set in the amulet was originally green and its examine was The amulet is green. Upon release of the Underground Pass, the stone changed to red and the examine changed to The amulet is red. Since the release of the Underground Pass, it serves no purpose other than as a cosmetic item.
The amulet was available as a reward for a while in RuneScape 2 even though the Underground Pass quest had already been released. It was called Lathas' amulet and was very similar in appearance to the Pendant of lucien and the Armadyl pendant. If the player had one when they were made discontinued they kept it as a new but untradeable Discontinued Item.