the mission isnt always the vlack garden, its a random story mission thta involves killing some kind of elite enemy at the end of it at Level 24, with Epic difficulty and 2 random modifiers on it.
for me and naturemon, we did it three times and got the first mission in the game, Gatelord Killing mission and First mission again.
Naturemon got the robe all 3 times, I got Helmet twice and chestplate once.