Hi : looks like Blizzard “modified” the event after the PTR… by adding a super-uber (Diablo)… and a really tough one too.
There’s an epic account of a fight (in French) here:
The story is a bit much for Google Translate but it sounds like a lot of fun 
The « reward » for finally killing Diablo appears to be crafting materials for an hellfire amulet 
The author says it’s not worth the trouble.
@KomisarzP : until you reach Trust Level 3 or Blizzard reduces the requirement to display a link in a message, you can use this trick to post a link:
- copy-paste the link as you would normally,
- highlight the entire link, and
- click the </> button on the formatting bar at the top of the window.
Your link will look like this : https://imgur.com/a/XXawiaG and it won’t be clickable… but the forum software will allow it. And, most importantly I think, others can copy-paste your link in their browser.