In this sentence:
Alles was Weed tun müsste, wäre am Leben zu bleiben, da sonst die Quest fehlschlagen würde.
I would translate it with the omission of da:
…, otherwise he would fail the quest.
So I don't understand why there is an additional da.
"da" = "weil". Possible English translations would be "since", "as", or "because".
Da is not only a local adverb; it's a conjunction as well and means weil. See here how its use is geographically distributed. At least in Germany, particularly Center and North Germany, it seems to be quite common.
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2I'm rather surprised that it is uncommon in parts of Germany. It's completely normal - maybe "da sonst" is a little more colloquial than "weil sonst" - and you should find lots of examples in literature for this usage. – Zane Mar 14 '14 at 19:49
It could be translated as since. Together with sonst you could say since otherwise.
See also here.
About the optional nature of „da“ in the original example: There are often ways to omit conjunctions.
What may be relevant to consider:
Alles was Weed tun müsste, wäre am Leben zu bleiben, da sonst die Quest fehlschlagen würde.
would become
Alles was Weed tun müsste, wäre am Leben zu bleiben. Sonst würde die Quest fehlschlagen.
That is two main clauses (verb second).
As an example of the pathway leading to complex constructions, I often see two main clauses connected with comma + „deshalb“ ‚therefore‘. That is (still) wrong in Standard Written German but apparently many people (including myself) consider this a viable complex construction.
