Mature grotworms are found in the bottom level of the Grotworm Lair, which can be accessed by either a long trek through the cave, or a shortcut requiring 50 Agility. The grotworms are level 98 and have 10500 life points, so they are much harder to kill than they used to be. They are weak to bolts so it may be worth it to use Ranged against them, but a Magic weapon above level 70 (such as a Staff of light or Polypore staff) is also fairly effective. Consider using Soul Split or a bunyip, but the grotworms also drop two un-noted sharks commonly.