1) The closest existing lexical entries to 'immortal' are unslaad and unahzaal, both meaning 'unending'. Some possible bases for a negated compound are joor 'mortal' (noun) and viir 'dying, ceasing, having an end' (adjective). The customary negator would then be vo-, which is seen more commonly than ni-.

2) The strict form of a draconic name template allows exactly three monosyllabic words. Most should be able to act as nouns, that is, nouns or strong verbs or strong adjectives (Alduin, Paarthurnax, Odahviing, Vulthuryol, Mirmulnir, Viinturuth). Additionally, adverbs like neh 'never' can occupy the middle position between two nouns (Durnehviir). The atypical loose form allows four syllables, non-monosyllabic components, or both (Relonikiv and possibly Sahrotaar).

For an approximation of 'Immortal Blood God' within the strict template, I suggest Ulsosrah. Ul 'eternity' connotes something that exists at every point in time.

"why does Unslaad shorten to Ul- instead of say Un- ?"

Thanks for the catch. Ul is a completely unrelated, independent word from unslaad, though with a similar meaning. Click the words to view their dictionary pages. There is no 'shortening' in this sense in dragon names -- the way to get rid of polysyllables is substituting a different word, and here we got lucky.

(This is the same person, just got too wrapped up and forgot to make an account before)
Just wanted to thank you both for the swift and informative replies, there's so much to learn here!

So I like how the name ends up, I just wish to understand the mechanics: I see that a dragon name needs to be monosyllabic, but why does Unslaad shorten to Ul- instead of say Un- ?

Oh I see now, lucky indeed :)

Thank you so much for your time and knowledge!

Edits: A-h-a and A-h seem to keep changing to 4 with this text input... strange.