For the boss 'Nezic, the lich', the description should read as follows (rather than the default text for a lich):
A powerful undead creature that has clung to existence with dark magic, insisting with malevolent force that others feel the anguish it refuses to acknowledge for itself. Limbs with dried, tattered flesh beneath a terrible skeletal visage are barely visible from within a lush, flowing robe. The garment’s impeccable condition may be due to vain magic, or merely a recent acquisition from some poor soul who's life now sustains this once-man.
When you look at a boss monster and press [M] for a description, you will always read the generic description of the monster type. The special message is only displayed when you just meet him.