Cofagrigus is a ghost-type Generation V Pokemon, and it is the final evolutionary form of Yamask. While Yamask looked like a spirit holding a death mask, Cofagrigus looks like a golden sarcophagus with four spirit hands made from shadow coming from inside.
While Yamask was horrifying because of what it was, Cofagrigus is horrifying because of what it does, namely turn other people and pokemon into the undead. Cofagrigus pretend to be inanimate coffins, and when someone gets too close to them, they open themselves and devour whoever's unfortunate enough to be nearby. They then spit out whoever it is as an undead mummy.
This quirk actually extends into the Pokemon gameplay as well; when a pokemon makes a contact-hit against Cofagrigus (in that it physically contacts it as opposed to throwing fire or something), the ability "Mummy" triggers and gives the ability to the other pokemon, thus preventing the other pokemon from using any useful abilities they may have. It is a temporary effect, and will go away once the battle is over (though not when Cofagrigus is defeated. If you are in a trainer battle, and there are more pokemon to fight after Cofagrigus, then the ability will be stuck with you until you win).
The Black and White pokedex entries are as follows:
"They pretend to be elaborate coffins to teach lessons to grave robbers. Their bodies are covered in pure gold."