

More difficult content will have a higher chance of dropping these items ( Source).While the legendaries are rare drops, they are not fully random - there will be ways to target specific items ( Source).Blizzard may expand where they can come from in the future (Source: Legion Summit Dev Interview, Video | Text Summary June 16 Legion Live Developer Q & A, 53:30) As it stands, these will be the ONLY sources that will yield legendaries. Thus far, it appears that legendaries will drop from dungeon bosses, raid bosses, PvP Strongboxes, Mythic+ Weekly Caches and World Quest Caches (rewarded after completing 4 world quests).

They are currently item level 895, and it remains to be seen whether this will change prior to release, or during Legion itself (for comparison, Mythic mode gear from The Emerald Nightmare is iLvl 880).
