These achievements won't show up in your steam achievements at first
and will not have a description in the global achievement list. They are;

• It's a trap!
  
• Ouch!
  
• Do you want to know more?
  
• Karmic Justice
  
• So close!
  
• Cheat like it's 1994

It's a trap!

-"You've been ambushed in a treasure room"

You probably already have this one just by playing. When you encounter a
room with a mystery box, chances are it's a trap. (You'll know because of
a soundclip of an evil laugh playing and enemies suddenly spawning around
you). Just keep opening mystery boxes until you get this one. Shouldn't be
a problem.

Ouch!

-"You died on a danger room"

You get this achievement by dying in a danger room. Danger rooms are identified
by a spiky tile icon on the map. These can be yellow or red. (Red ones contain a
trickier obstacle course that rewards the player while yellow ones are fairly easy
and are just used to connect to other rooms.) Dying in any of these rooms will net
you the achievement.

Do you want to know more?

-"Find a secret room"

You'll get this one by finding a secret room. ditman made a handy guide for finding
them here! http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=308593527
There's one on each level and they are quite helpful so I suggest you learn to
identify them.

Karmic Justice

-"Died after killing the boss"

After beating a boss on floor 1-4 a portal will open taking you to the next floor. DO
NOT go in the portal. Instead, stay on that floor and kill yourself. Either by self
damage with grenades or elemental damage, spikes, lava etc. You will still get the
achievement if you kill yourself in other rooms while the portal is still up so you can
search for lava or spikes to kill yourself.

I suggest you start a game on easy, sprint through all rooms and finish off the boss
on floor 1 and then kill yourself.

NOTE: You will NOT get the achievement if you kill yourself using cheats.

So close!

-"You met your match in the final showdown"

When reaching the final boss on floor 5 you will get this as a consolation prize when
you die.

Cheat like it's 1994

-"Find a super secret cheat code"

Even though Doom came out in late 1993, you'll get this achievement by typing in a
doom cheat code.


Using cheats will nullify your leaderboard and unlocks, but other than that you can
continue playing. Cheats will affect only your current run.

saturnine commented:
"The Doom "cheat codes" do the exact opposite of what they're supposed to do.

IDDQD - God mode in Doom. Kills you instantly here.
IDKFA - Gives all weapons in Doom. Takes away all your weapons here."

Other know cheatcodes are:

MASSACRE - Kills all enemies in the room
CARMACK - Gives game an older pixelated look
RAMBO - Gives player weapons and mana
QUICKEN - God Mode
POUNCE - Give player 100% health
SHAZAM - Gain 1 level of experience

It is best to just type a cheatcode in the very beginning of your run, get the achievement,
and then just restart fresh. Or just have fun with weird cheats..

If you spot any mistakes or have suggestions, please type them in the comments below!