The Elder Sandworm made its debut appearance during the Animal Invasion special event, and was then added to the pool of Critter invasions, albeit it is a much rarer occurrence, after the conclusion of the Raiders vs. Bottle Rockets special event.
The Elder Sandworm is truly an impressive looking beast. Its attack is extremely dangerous to all units and defensive structures, it is giant vomit bath that hits 3 squares for over 250 damage normally; upward of 400+ if critical.
The Elder Sandworm can only attack the 3 squares in front of it, up to two rows away. Placing a single unit off to either far sides will render a lone Elder Sandworm helpless.
There is a second version of the Elder Sandworm encounter, which boasts a much stronger Elder Sandworm with 1100 HP and armor (as opposed to it being alone, which then it only has 450 HP and armor). In this case, the Elder is accompanied by 4 other sandworms, two Groundshakers and two Sandworm Grublings. As long as players keep their units out of the Elder Sandworm's attack range, the battle can be won with very little or no causalities.
A third Elder Sandworm (lvl 45) encounter also exists once you reach level 42. It consists of a pair of Elder Sandworms with no other sandworms to back them up. This encounter is fairly simple if you plan your formation correctly as you get a Big Rock that you can place in the front row that is impervious to poison damage that the elders dish out.