The name of this dwarf has long since been lost to history. All I remember is the nickname I gave him afterward. This story took place in 40d.
I built my fort over an underground river. When I first started the fort, I opened a well over it for water, at which point an olmman jumped out and ate my cook. I quickly walled that original well off and built a new one far away, connected to the underground river by a long section of piping and fortifications to prevent stuff from going through.
Soon after, my fort prospered and grew. Fifteen years passed, and my founders' children were now adults, the Duke and the rest of the nobility had arrived long ago, and an elven caravan was walled up behind the trade depot, surprisingly not yet having gone insane despite being trapped in the darkness the whole time.
Suddenly I get the message that one of my founders, the mayor and chief legendary miner, has bled to death. I quickly zoom to her corpse, where another couple of dwarves are being savaged by a giant olm. As soon as those two were dead, it turned and vanished down the well. I checked one z-level lower, and saw that despite the precautions I had taken, a giant olm had somehow squeezed through the fortifications and entered the plumbing. Now it was hiding there, waiting for more dwarves to pass by so it could murder them.
Now, these were not vanilla weakling giant olms. Although my giant olms lacked building destroyer to preserve FPS, they made up for it by being [SIZE:20] and not being able to feel pain.
I quickly ordered my military into action. I didn't have many champions at this point because many had died in the orc siege last season. Only three - a grizzled, one-lunged veteran, one of the Duke's grown-up sons, and the son of my armorer (a founder) - were healthy enough to answer the call.
The veteran and the duke's son reached the well, but the armorer's son lagged behind. Cursing him, I focused my attention on the other two. The duke's son emptied his crossbow at the olm, succeeding at lodging a bolt in its long torso, after which it bit his head off. The veteran got his sword stuck in the olm and wrestled it into the water, where they fought for a while, bloodying up the river, before he drowned.
By now the olm had a full-blown title, and the armorer's son was the only one left. He didn't have any kills to his name, and I didn't think he would survive a battle, so I changed his orders so that he would be stationed a short distance from the well, hoping that his presence would be sufficient to frighten off the olm. Instead, he dived down the well with his steel blade held high.
Horrorstruck, I zoomed below, where he and the olm desperately battled each other in a cloud of blood. I checked the status of the olm, and saw that it was actually covered in wounds, while the armorer's son was uninjured. Against all odds, the armorer's son killed the giant olm, and then returned to the surface unharmed before going to get a stiff drink. The olm's corpse stained the banks of the river red that season.
Later on, he went on to marry the daughter of the duke, before he was killed by fireball-throwing goblins. The nickname I gave him for this deed is written on his tombstone: Olmbane.