Vessels are the neutral term given to a Piece of Eden. They come in several forms including swords, ankhs, Apples, and others with the most common being Apples. Each variation of a vessel was designed for something. Each one contains the soul and mind of a pre-hibernation angel and they are forced into service by the vessel.
The angels within these vessels are beings of intense hatred and rage and will normally corrupt their wielder or drive them mad. This is most noticeable on humans, who feel their pain so much more strongly and become obsessive of their vessel, wanting to protect and care for it so that nothing can hurt it like the vessel was hurt before. Vessel angels desire a strong connection like they had in life with their other fellow angels so when a modern one comes along they will possessively manipulate them. This leads the modern angel to wanting to protect and obsess over the vessel angel even more.
It can happen to proeathans but they need much longer exposure to the vessel to see the same effects where a human will see the effects within a few weeks or months.
After the end of TSW all pre-hibernation vessels were destroyed by the seventeenth stadalla Desmond Miles. The use or creation of a vessel is punishable by Fold law that will end in jail time and an incredibly steep fine. Multiple offenders may even face the death penalty for vessel induced murder.