You get the weekly dungeon keys by gathering fragments, these fragments come from opening a daily dungeon Key
This dungeon is harder than he previous one (but not much) and must be played with natural 3* characters.
It opens for two hours and can be farmed for that time.
The sand of time cost is steep, so be prepared to spend ironite on sands to actually profit from this dungeon.
The rewards are slightly better (slightly being the operative word here) than the daily ones.