Why I encountered Diablo in The Forgotten Tower in Diablo II?

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.

All of the information about Uber Diablo can be found here

The PlugY mod has a built in function to spawn Uber Diablo at random intervals. Usually Uber Diablo only spawns on multiplayer battle.net games, but since that's not possible on single player, PlugY just randomly does it.

The fact that he spawned in The Forgotten Tower, is because he will always spawn and the first Super Unique enemy you encounter. More information about Super Unique monsters can be found in the Uber Elder link at the top of the answer. If you want him to go away so you can complete that quest, save and quit and go back into your character. He will despawn from that location and randomly appear whenever PlugY decides is the right time for him to come back.