This ghostly duke is encountered in the Tower of Evermore. There is also a 1 in 100 chance of encountering him in Cordova Town or in Eneomaos Machine Tower (similarly to the Golden Bones in the Baljhet Mountains).
He has the abilities of both a Dead Baron and a Dead Fencer, and is considerably more lethal than either because he is vastly more powerful than the counterparts he can potentially appear alongside in Cordova. An unfortunate low-level character that runs across him can quickly find themself staring at the game over screen.
There are several reasons to pursue him, however. He has an entry in the Bestiary, offers a fair amount of stealable money ($500), and has the ability to drop the coveted Light Ring, which reduces Holy damage (and he has a reasonable chance to do so).
Note that unlike Dead Barons and Dead Fencers, the Duke Mirage will often delay his attacks and use feints in the middle of combat, making him considerably more irritating. He has a tendency to hang back and observe the player before attacking, hinting at him studying their moves.