"I can't believe that just happened..."

[Joccaren]

Dwarf Fortress has a lot of those, like a dwarf mother who uses her own baby to construct a dwarf-bone bracelet or something. I've also read stories of a duck who killed two powerful creatures on its own and gained a title!

Oh indeed. Dwarf Fortress is hilarious in how things like that happen. Just Google Boatmurdered. Seriously, the shit that goes down near the end of that is... WTF.
And then there was one of my Dwarves who engraved an engraving of an engraving of the engraving on a legendary spear that was an engraving about the engraving on the wall of my Throne room, which was an engraving of the first engraving in my fort. That's right, my Dwarf Engrave-ceptioned about 5 levels. That first engraving was about cheese or something.

Oh, and ducks aren't what you've got to be careful of. Them common snapping turtles... Best guards your fort can ever have.

Really, it makes more sense when you understand how combat runs in Dwarf Fortress, and why the Common Snapping Turtle's beak counts as a highly effective weapon that lets it decapitate an army of goblin raiders, but even then its still very WTF when that Kobold Theif that tried to steal your treasure has his leg bitten off by a common snapping turtle. Oh, and then the Ogre that attacked the common snapping turtle;
Common Snapping Turtle charges Ogre.
The Ogre is shocked by the Common Snapping Turtle's ferocity [Basically means he misses his combat turn].
Common Snapping turtle bites the Ogre on the head, the severed limb goes flying off in an arc.

Then there's my Sparta dwarf, who got into a fight with a Goblin in the Cave system under my fort, and kicked him down a 10 level deep hole when the battle reached its climax.
Or half the enemies that don't die, but are fatally injured by a bolt or something from one of your archers. I had this as my combat log for a Goblin my archer shot;

Spoiler

The Silver Bolt hits the Goblin in the arm, fracturing the bone.
Goblin Wretches
Goblin Vomits
Goblin Wretches
Goblin Vomits
Goblin Vomits
Goblin Wretches x3
Goblin Wretches
Goblin Vomits
Goblin Wretches
Goblin Wretches
Goblin Vomits x2
Goblin Wretches x3
Goblin Vomits
Goblin Vomits x3
Goblin Wretches x4
Goblin Wretches
Goblin Wretches
Goblin Vomits x5
Goblin Vomits
Goblin Wretches x3
Goblin Wretches x2
Goblin Vomits x5
Goblin Wretches x3
Goblin Wretches
Goblin Vomits x3
Goblin Vomits x2
Goblin Vomits x3
Goblin Wretches
Goblin Wretches
Goblin Wretches
Goblin Wretches
Goblin Wretches x5
Goblin Vomits
Goblin Wretches
Goblin Wretches x4
Goblin Vomits x2
Goblin Vomits x3

-ect. ---> That was only the first of 5 pages, with the same set of messages on each. Took a good year for that Goblin to die, whilst he was unable to move thanks to constantly throwing up.

Also love the times you think your fort is screwed, but it turns out Armok is on your side this round. Army of Ogres coming up from the caverns, chasing my miner. I have no military ATM 'cause I've struggled to find any metal to make stuff with. Apparently mining skill aids in the use of a pick as a weapon, though by default your Dwarfs won't use picks as such. Miner gets cornered, and has to fight. Instant headpicks on 3 Ogres, instant deaths to them all, then he just goes off and starts to dig a whole again.
Or when the fifth consecutive Mega Beast came to my fort [Only one can attack your fort at once if memory serves, but they were all just waiting so one died, and the very next frame the next would come in]. My military was tired and worn out from a month of fighting without food or drink, and about to feint and die to this stupid Megabeast, becoming the first casualties of my fort. Nope, Goblin invasion. Goblins come across the Megabeast. Megabeast slaughters goblins, but is heavily wounded. My best swordwarf charges up to it and decapitates it instantly, gaining his 5 Megabeast kill and 24th named creature kill for that fort.

Seriously... Dwarf Fortress... Too much WTF stuff to describe.

 

[Joccaren]

They were demons, in hell. So the fort made a duck launcher giving how effective they were.

Hmm. What type of demons?
One of my favourite things about demons in Dwarf Fortress is that they are procedurally generated, and completely random. Because of this you get some OMGWTFBBQ pwnage ones [Like three headed Snakes that shoot flames and webs that have wings and can fly, and are made of steel], and some absolutely trollingly pathetic ones [Like ones made out of air that can just push your Dwarf, whilst your dwarf just hits them once [Which is admittedly hard considering they're made out of air] and they die. One of my favourites was a Demon King of the Goblins [For those who don't know, at world generation some demons are spawned in the overworld, outside of hell, and can become the King of a non-player race from time to time] who decided to invade my fort with an army of about 140 Goblins of assorted military professions, all riding Cave Crocodiles. I thought I was screwed, they could even swim across my fort's moat!
Nope... Demon King charges forward to cross my moat... Disintegrates. He was made out of salt and the water dissolved him. His entire army about-faced and ran for their lives after that. I was just like... ... Really... Demon King made of salt thought it was a good idea to try and swim, without sending his army in first?