Well... as for Azmodan, I think this is a good start for concepts.
http://www.wizards.com/dnd/images/bo...D_WP2_1024.jpg
Belial on the other had could be a bit more diverse looking.
(this guy is named belial, so obviously this was my first choice)
http://www.wizards.com/dnd/images/bo...161_620_46.jpg
This is my second pick.
http://www.wizards.com/dnd/images/bo...161_620_50.jpg His name is mephistopheles... but he fits your description of belial fairly well
This is my favorite pick for belial... and it matches your description quite well Hanni.
http://www.wizards.com/dnd/images/bo...161_620_40.jpg
Hope these pictures can provide some inspiration.
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