After the Mud Pack, Preston Payne and Sondra Fuller fall in love and eventually have a child together named Cassius "Clay" Payne, who became the fifth Clayface who debuted in Batman #550. The boy is separated from his parents and held in a government laboratory. The full extent of his powers are unknown. The name "Cassius Payne" is a pun on "Cassius Clay" the birthname of legendary boxer Muhammad Ali.
If a piece of him is separated from his body, it can grow a mind of its own, but it mostly thinks in an unstable form of what Cassius wants. If bonded with another human, becoming a "Claything", the piece can give that human Clayface-like abilities, such as becoming soft and malleable, being able to withstand bullets and other harm, and could also manifest Payne's ability to melt objects; all this person would have to do is just think it.
In an issue of Batman: Gotham Knights, Cassius was depicted as having the clay-like appearance of his mother and father, but can only stay in Clayface mode while awake.
Cassius was featured in Final Crisis Aftermath: Run #3.
Powers and Abilities
Cassius seems to possesses the same clay-abilities as the other Clayfaces: a clay body, shapeshifting, etc. In addition, he can split a piece of himself off, and it will think independently (though mostly in an unstable form of what Cassius wants). If the piece bonds to a human, that human becomes a "Claything"