My New Favorite D&D Monster

Derek Holland
The nature elemental was introduced in The Ruins of Zhentil Keep (which I do not have) and reprinted in the Annual Monstrous Compendium Two (which I do). This extremely powerful creature is a weapon that removes all traces of  civilization within a certain area and replaces it with the natural setting that existed before. So in effect it is similar to the Genesis Device in Star Trek II. The elemental is difficult to destroy as it draws on all four elements and  can regenerate all damage it takes per round as long as it is in contact with any of the elements. Like most AD&D monsters, it has a lot of wiggle room in how a DM can interpret its powers and effects. For me, I think of the nature  elemental as an eraser that only applies to artificial objects and a restorer of natural materials. So if one was used to destroy a mine, the seems of ore would be restored along with the bedrock and any forest, meadow or hills on the surface that was damaged or destroyed by the miners. In itself the nature elemental has a variety of uses, from destroying massive walls preventing invasion to wiping out an orc city state so that elves or humans can replace them entirely.
But those are the only use for the critter. There can be variants that change the resulting environment. Celestials could have their nature avatars that not only create forest from ruins, but infuse it with the power of goodness to assist the new comer colonists the angels lead to these lands. Aberrations could have unnatural avatars that strip away swamps and replace them with pools of goo or mountain tops with strange metallic bubbles that support alien life. The possibilities are endless. Moreso when the powers that use such elementals use them on a regular basis to wipe each other out. The land would become a patchwork of varying ecosystems, each with their own complement of monsters the elementals would drive out when they work. The more they are used, the more people and monsters would be slain as well, so the war would be slow and constant even with heroes on all sides. Cities would have to be protected by massive amounts of magic, otherwise they could be erased from existence and many of their populations slaughtered with a single casting.
As this could get overwhelming for a DM (replacing city maps and entire cultures every so often can do that), one simple limitation is to make the summoning costly. That keeps the players from using the elementals as weapons of convenience and leaves them as the plot drivers they are meant to be.