This video will show you Resplendent Chest in Cathedral Level 4.
As you can see, this isn't special quest or event. I uploaded this video because lots of beta players miss this and run straight to SK.
I remember there is one special event in Cathedral. I'm still searching for that one.
(If you know what floor it is, please share your valuable information with me)
Anyways, I've found one rare drop from this chest. If you haven't found this chest or tired of doing SK run, try to find this chest for fun :)
Resplendent Chest - http://www.youtube.com/watch?v=gzBSvPn1C3Q
Diablo III Playlist - http://www.youtube.com/playlist?list=PL802A63587EC10ADB&feature=plcp