Or that she is psion with access to<em>metamorphosis</em>power (yuan-ti are psionic creatures and expanded psionic handbook has more details about it). Five levels not quite enough for get said power but she could have magic crystal to allow it.<br/><br/>Or we simply rule the 'alternate form' power what all yuan-ti seem have is metamorphosis power (which is limited to form of vipers but through unique dealings your abomination can use said power take humanoid form instead).<br/><br/>Anyways that lamia girl/human forms work quite fine and there is lots art about her :). Anyways do remember my guy is closely aligned with hell, so our lamia/yuan-ti abomination is not chaotic evil (it may come CE society but she had changed to LE alignment over time and yuan-ti overall look more a LE society than CE one how organized they are).