Challenge doesn't have a chest, the end rewards have a chance to reward you a 335 ghost, class item, or one of the three PoE exotics. The sigil that you get from variks can also give you one weapon up to 335 and one armor up to 335.
Edit: high score gets you the weapon and cumulative score gets you the armor
Thanks for the heads up. That seems like a weird system. I'm not exactly sure how everyone's light levels are flying up. I've done the following on this character and not really moved: entire Variks quest, PoE 41, Trials Cards x2, Nightfall.