Unrest is a good place to solo, many different levels of rooms and with Lull drawing singles from rooms is pretty easy. you can solo here to 40 if you so deisire but i left at 34. My next step was Lower Guk another Undead haven but with more live pray too such as Bats and Spiders. Lower Guk is a kind of higher level unrest but very dangerous. However both places are great for groups when you tire of soloing. Another good place is Kithicor forrest, more open for Undead soloing and advisable to stay near zonelines to Rivervale or High Hold Pass as adds are fast and furious. The undead however only spawn at night in Kithicor.
After this, although not undead i soloed the Tundras after hitting 50, Eastern wastes for Manticores, Snow griffons, Wooly Rhinos..list is endless. The Shardwurms in GD drop nice spells and then Velketor's Labarynth Just of GD can see you solo upto level 60, if patient from the Ground floor mobs. Also the Velk mobs drop Bricks of vellium and crystaline silks, which sell very well.
After 60 The land of Norrath is open to many places, however teaming for the right equipment along the way is essential.
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'The only thing I cannot resist is temptation.', Oscar Wilde.