Is the muse actually a lich? If so, why hasn't Tarek attempted to smite yet? (Besides the metagame "the blogs are primarily about the game, the actual RP is secondary" – unless that's the answer.)
(That pretty much is the answer, but also,) I figure Tarek @wearepaladin has more… immediate things to do? Obviously Tarek and a centuries-old lawful-evil lich like Sethos would be at odds, but Sethos spends most of his time just kinda chillin’ like a villain nowadays. He conquered the kingdom around his lair a very long time ago (probably before Tarek was even born); the area’s turned into an uninhabited (sans undead) wasteland for miles in every direction; and Sethos can go hundreds of years without leaving his necropolis or really bothering anyone. It’ll happen eventually, but I would think Tarek has more immediate things to worry about.