The Ascidians were an alien species planned to be included in the cancelled X-COM FPS game, X-COM Alliance. They were assisted by X-COM, and helped their Human allies battle the fearsome threat who had returned after their defeat in Enemy unknown.
When an X-COM crew, stationed on the lost ship "UGS Patton" accidentally flies through an inter-dimensional worm hole, they are shocked to find remnants of the alien creatures from Enemy Unknown inside. The alien menace, who had once before threatened humanity in The First Alien War, had turned their sights to the Ascidians, the natives of this other realm. Humanity, who had been in the Ascidians' position many years in the past, decided to help them, and so the alliance between the Humans and Ascidians was born, promising to cleanse the hostile alien threat from the Ascidian dimension.
Unfortunately, that is all the information we were given about them. Technology, appearance, history and behaviour, none of this was ever explained, we can only make a few assumptions based on the given information.
The Ascidians have been given no official picture releases or description, but the name "Ascidian" provides us with a clue. In reality, Ascidians are a type of sea creature, more commonly known as a "sea squirt". While these creatures would probably make for boring and unhelpful allies, the Ascidians could perhaps derive physical features from these deep dwelling growths, perhaps sporting some sort of growth on their bodies resembling these creatures, or simply being of aquatic origin.