I was playing Diablo II LoD recently with sorceress on Hell difficulty. Unexpectedly, when I got to The Forgotten Tower, instead of The Countess I encountered Diablo boss. Was not prepared for that and died. When going there second time everything was OK, The Countess was there. I haven't found on the Internet any information about this. All I found was Uber Diablo and Pandemonium Diablo, but it seems that they should spawn in another location.
Why Diablo was there? Is it some event, bug or maybe easter egg? I am playing with PlugY mod, maybe it did spawn Diablo? However, I haven't found anything related in its configuration file.