The beings known to humanity as the Elders after the signing of the Great Accord in July 2015 are ostensibly its greatest benefactors. Incredibly powerful psionic beings millenia old, the Elders have journeyed across the stars, bringing peace and prosperity to the races they encounter, integrating them into their vast empire. Although rarely seen in public and virtually unseen and unheard from since the Accord, the Elders remain present in the public consciousness as physical, material gods that have displaced all elder beliefs - or so ADVENT claims.
In reality, the Elders are selfish parasites that conquer entire worlds in search of a cure for themselves. The entire ethereal species is uniquely afflicted by a form of rapid muscular degeneration: Their physical forms are no longer capable of containing the Psionic energy that is the Elders' lifeblood. All the servant races are those that the Elders have encountered and used in an attempt to cure this plague among their own kind. Each time they encountered a new species they procured what genetic information and material they required and moved on to the next world. They've never found a cure, until finally reaching Earth. 
After centuries of searching for a solution, humanity's genetic makeup and compatibility with the psionic gift has shown tremendous promise, resulting in the Avatar Project. Rather than simply conquer and assimilate, the Elders created ADVENT to produce a benign facade and draw humans to population centers, to harvest their genetic information and create a perfect host body to replace the ethereals' frail old forms. The resulting Avatar would be custom-made from templates created in the course of the Project, combining psionically charged human DNA combined with that of the Elders, and allowing each to possess the Avatar.
According to the Chosen, the Elders' prized hybrid children, the Elders are searching for a cure in order to counter an even greater threat to the very universe that lurks in the shadows and will eventually pose a clear and present threat to life in the universe.
The Elders are completely alien and view humanity as little more than means to an end. It would be incorrect to classify them as evil or good, as their morality spectrum is utterly foreign. All races that serve them are seen as instruments to use in pursuit of their goal and to advance their agenda. If that means killing men, women, and children, and processing them by the thousands into genetic raw materials for the construction of their Avatars, it's considered perfectly acceptable.
However, they seem to possess some variant of empathy or compassion, as they are fond of their Chosen and when they are killed, they are clearly disturbed by their loss.
- The Elders are not directly confronted in XCOM 2, but serve as behind the scenes antagonists manipulating events and the world. The closest XCOM 2 gets to fighting them is when they confront the Avatars on the ground, however you do confront them in Enemy Unknown late in-game, acting similarly on the battlefield as sectoids. After the attack on the Alien Fortress beneath the Pacific Ocean, the Elders are effectively wiped out.