The only unusual message it'll tell you is if you didn't pick up the last rune. It'll remind you that you forgot something important.

Once all four runes are obtained a hole in the floor will open in the start level. This will lead to the last boss.

I do not recall a nightmare difficulty tease. Proquake addition? Probably. I never used that engine.

Nightmare has always been there ;-P You'll find it on the entrance to the 4th episode (be careful when jumping down through the pool of water 😁

yeah nightmare mode is hidden a bit, fall into the water but hug the wall so you land on top of the beam, then you can follow it around to a hole in the wall which leads to nightmare difficulty.

as for the teaser thing, this is definitely a proquake thing, and not in the dos or gl version of quake, nor is it in dark places (the engine I prefer).

Proquake is certainly messing with you 😜
Not that you're missing much, the final boss/level ain't something spectacular... I highly recommend you play the two official mission packs if you want something similar to the Quake SP.
Especially the first one is very good, the second one is ok for the most part, but it just doesn't add anything new other than variations on the enemies and weapons.

I've been playing the original Quake games, plus the two add-on packs as well. I kind of did them out of order- I beat Quake and Dissolution of Eternity, but didn't complete Scourge of Armagon yet. I got all the way to one of the last levels and then ended up somehow deleting my savegame. I really loved the laser gun add on.

It's been a while, but as far as I can remember, once you get the fourth rune, it's over - back to the opening. I don't remember any end boss.

Quake 1 final boss:

I have been playing Quake lately - I will have to see this for myself. Like I said, it has been a long time....

Thanks all.

I know about the location of Nightmare mode but didn't think it was right to be sent there after playing on easy!

Luckily I didn't overwrite my savegame of the last few levels. This time using GL Proquake with that save I was able to get to the final boss which like alot of games is a let down. The Win ProQuake api must have some bug to punish me like that. Maybe I'll see if it repeats the issue with that client.

I wasted so much ammo on Shub-Niggurath when I first got to her/it. "Why won't it die??" 🤣

I did the same damn thing the first time on that level. Used up all my ammo before figuring out I had to basically telefrag it to kill it. Lots of gibs bouncing during ending cinamatic. Used original quake, never did care for GL quake.

I tried that Tenebrae engine with Quake once, made it look a little too much like Doom 3, IMHO. It was kinda cool though. Every time I use GLQuake, I get a flickering Hud display where the numbers for your ammo and health are. I was also running it in 3dfx mode, so maybe it just doesn't like how the drivers are set. I agree though, classic Quake is the bomb! The way Id meant it from the start.

I went through the whole game with DarkPlaces and got to the boss and I was like "What the heck is this thing?" Quake was freaking weird. Too weird for me. Never liked it.