Not actually undead, though from the looks of his condition you have to wonder if he's still technically alive.
Was once an artisan watchmaker and repairer of clocks. Something must have happened between then and now-- you don't just turn up one day looking like a living embodiment of death. No one really knows his story though, probably because no one ever gets close enough to ask. Radium exudes feelings of instability and danger and he certainly looks the part. Even the most staunch of characters find it preferable to flee when he makes an appearance. His movements are swift and erratic, his eyes wild, and he seems perfectly capable of running you down and ripping your face off. If you ever need a watch repaired, it's probably best to ask someone else.