Okay... The original guide wasn't very good so I'm gonna try again... Sorry for the rush job last time, I was at school and BORED OUT OF MY MIND... anyway...
First... I got asked: how do you get there? Ok... In level 15 you will find a Lazarus staff or staff of Lazarus or something (it's been a while)... Then take it to deckard Cain and he will tell you a little info about himself. Then you can find a red portal to a place where you kill the archbishop Lazarus. After you kill him, the pentagram seal will open and those are the stairs to level 16. That's how it was on ps1 anyway... Tell me if it's different on computer.
Then diablo is on level 16 so be ready. If you're playing on playstation... Buy as many full health potions as you can, unless you're a sorceror, the buy full mana potions instead and cast mana shield.
You have to pull a bunch of levers to get to diablo as well.
I will post more as I do it but all of my characters gotdeleted so it might be a while... Good luck!