If Umber is stealthed can he use his own ability to join the challenge after defenders have been declared? Of does stealth lock him out of the entire challenge?

Rather than type it out again, I'm going to refer you to this thread .

thank you kindly. I seem to have made a newbie error and failed to properly search the forum before posting.

No worries. It's not a particularly easy forum to search.

It's an AWFUL forum to search -- if you use the built-in search function. But if you use Google, you can get decent results. You'll get the best results if you type "site:www.fantasyflightgames.com" before your search term in Google.