The Rule of Two

The Rule of Two is the unwritten code that states that, "Always two, there are--a Master and an apprentice," as Yoda observed.

Remember: power alone is not enough. Patience. Cunning. Secrecy. These are the tools we will use to bring down the Jedi. The Sith are only two now--one Master and one apprentice. There will be no others.
This Darth Bane said to his apprentice Zannah.

The Sith Code is as follows:
Peace is a lie; there is only passion.
Through passion, I gain strength.
Through strength, I gain power.
Through power, I gain victory.
Through victory, my chains are broken.
The Force shall free me.

The users of the Dark side of the Force can undergo serious physical changes, including the change of the eyes from their normal color to an acid yellow. The skin, as in Palpatine's case, can also become gray and overly wrinkled. But these users can change between their original appearance and their current appearance, as Palpatine and Anakin have done.