The word in question is "thraal", a species from the Dr. Who universe (http://tardis.wikia.com/wiki/Thraal) and coincidentally also a species from the Transformers universe (http://tfwiki.net/wiki/Thraal).
The only similar word I know of is "Baal" and I've heard "bal", "bail" and "bawl" for that as well... so that doesn't help.
Any guidance?
Not only is the aa unclear from the spelling alone, so is the th. It could be /ðreɪəl/, /ðrɑːl/, /ðrɑːˌɑ/, /ðrɑˌɑ/, /θreɪəl/, /θrɑːl/, /θrɑːˌɑl/ or /θrɑˌɑl/ just going by the received pronunciations of the words Baal, Salaam, aa, the and theme. This is not exhaustive.
The association between spelling and pronunciation is a mixture of spellings and pronunciations mutating in semi-separate ways for the last 1000 years, while new borrowings keep being thrown in, with different degrees of Anglicisation, all the while.
Since the only source seems to be a novel, it's in fact quite possible that there isn't a pronunciation at all. They could very well have picked the unusual aa and the ambiguous th precisely because this doesn't give us enough information to make a confident guess as to the pronunciation. Indeed, given that the author has written at least one sci-fi-comedy novel, it's quite possible indeed. You could always email them and ask, but until then a conclusive answer is impossible.
[æ, e, eɪ, ʌ, ɐ, ɑ, ɒ, ə] there, although I don’t claim those are all phonemic distinctions for all speakers. Point is that there are a heck of a lot of them, and the rule is there’s no rule. (Ok, I wouldn’t expect aa to ever show up as [ʘyːǂ], but you get what I mean.)
While not actually disagreeing with any of the above, I think there are ad-hoc rules for pronouncing 'aa' and 'th' in unfamiliar and fictional names. 'Aa' will generally be assumed to be as in aardvark, and 'th', although it can be a voiced or unvoiced fricative in English generally, will always be assumed to be unvoiced in a new name.
Based on transliteration of Indic languages, I would assume /ɑ:/.
