Diablo 1 HD Hellfire Mod | Awake

As titled, I've just managed to kill Diablo in Belzebub HD mod.
After that, I went to Lazaru's Liar to attempt finishing the game to unlock higher difficulty but I am now stuck, and I am in need of your help.

I entered the liar, and a cutscene played showing Lazarus' eyes turning red.
I slaughtered everything in the lair, but I couldn't interact with Prince Albrecht's deadbody to get the soulstone.

May I ask if the game has bugged out at this point or if I had missed out something obvious? Please, I really need your help to unlock higher difficulties.

The boy in Lazarus's room is not Albrecht, Cain i believe tells you this once you kill Lazarus and come back to town to end the quest. So you must enter the pentagram (just below red portal where you go beat Lazarus) and there in Terror's Domain you will eventually fight Diablo and after you defeat him his body is that of Prince Albrecht, from which you get the Soulstone.

I've just managed to kill Diablo in Belzebub HD mod.
After that, I went to Lazaru's [Lair]

Well that's weird… I might be wrong but I was pretty sure you couldn't even get to Diablo's level (what it is called again, the Terror Domain or something?) before finishing the quest with Lazarus (which turns the pentagram/pentacle/however that star with 5 thingies is called… to red).

He propably thinks of Tchernobog, not Belzebub