If you reset your quests and play the game all the way through to Diablo and kill him, you will get a guaranteed legendary from his death. That, when playing alone, is simple enough to understand.

But how does it interact with other players who may or may not have also reset quests?

Here are a few examples:

1. If Player A resets quests and Player B does not, and they both play through every quest together, does Player B get a guaranteed legendary?
2. Does the answer to the previous scenario change if Player B only joins during Act 4, and doesn't play through himself?
3. If Player A resets quests and joins a game in progress from Player B, who did not reset quests, in act 4, will Player A get a guaranteed legendary despite skipping 3 acts?
4. Does the answer to the previous scenario change if Player B *had* reset quests and got this far on his own?

What are the **exact** conditions required to get a guaranteed legendary?