The Elder Druid seems to be the most senior Druid in the Druids' grove in Chapter II. It is from the Elder Druid that one can purchase a variety of items, including herbs, books and scrolls. In Chapter III, this position seems to have been taken over by the Hierophant.
In Chapter V, there are several druids going by the title of "Elder druid" in the druids' cave in the swamp cemetery. They are accompanied by an Eldest druid and are preparing for The Green March. They are interested in trading potions for a number of items to help with the march.
The Elder Druid typically buy and sell the following: