Spectral Cat

A Spectral Cat can be found sitting atop a column at the entrance to the Hall of Sorcery in Maldraxxus. It has the same model as Mr. Bigglesworth (most noticeably shown by its glowing icy whiskers), implying that it is Bigglesworth's soul, united with his master Kel'Thuzad (who can be found elsewhere in the House of Rituals) in the afterlife.