Wilhelm hints that the role of the Testaments involves "influencing space-time anomalies"; that much becomes clear when they resurrect Albedo and help to create a space-time anomaly which engulfs the 脵 System.
The Testaments are incredibly powerful and may easily rank among the most powerful entities in the series. Testaments are not corporeal in nature, in fact they are more like Gnosis (which they can also transform into) than they are human. In fact they do not even exist on the same plane as physical human beings. It is explained in the Episode III database that they only appear human because they affect people's perceptions, so much so that the people they confront actually believe they are seeing them as they are. An example is given that if a picture/video were taken of a Testament, they would not appear in it because they do not physically exist. However, if they wished it, they could cause themselves to appear in the photo/video because they are capable of manipulating the Real Numbers Domain from the Imaginary side.
They exist in only one of the two planes of existence the Xenosaga universe is composed of. A plane by the name "Imaginary Number Domain" (The part of the universe consisting of consciousness), exists parallel and yet in the same space as the plane known as "Real Number Domain" (The physical, material realm). Testaments exist within Imaginary Space. They are capable of continuing to exist as themselves beyond death, due to Wilhelm's God-like power. To be a candidate for Testament-hood, one must possess what Wilhelm calls the "Shining Will." It is revealed in the Episode III database that those who can become Testament are those whose wills resonate purely with Anima. Those who resonate purely with Anima do not transform into Gnosis, and are capable of awakening the Anima Vessels. The Black Testament's speech confirms that the main heroes were in fact all capable of becoming Testaments. Assisting in the awakening of the Anima Vessels for the sake of Zarathustra, is the main task of the Testaments.
Because they do not exist on our physical plane they can subvert the laws of physics to perform phenomenal feats. Virgil aptly explains; "The laws which govern us are just too different," after he easily brushes aside the party.
That's what wiki has about them, but after researching about them on cut-scenes that's pretty much what they.
Blaxican does not point out errors in people's posts and laugh without making any on-topic post. That would tend to annoy people, don't you think? And even then Blaxican got told in the Bryan Fury vs Voyager thread.
Are you going to throw a tantrum Guilty? Go ahead. Just don't hurt yourself in the process.
Tantrum? Absolutely not. You don't see me writing in caps, cursing, or posting smilies suggesting that I'm angry. I'm just pointing out a ridiculous double standard here.
Has he done that before? Maybe yes, maybe no. Do I tend to do it? Of course not. I did it that one time, and you chose to make a deal out of it, and in fact, that was made to make you laugh somewhat, not to annoy you, since I had no intention of doing that in the first place.