My 1 in 1,000,000 is more a person than an event.
I had, in a previous fortress, a superdwarvenly tough champion axelord, we will call him Urist McBadass, who had singlehandedly fought off three (non-bow) goblin sieges. One day he was patrolling around some goblin gear (I have to have him patrol near gear before letting people retrieve it due to the frequency of werewolves and undead) and it just so happens he is close enough to the underground pool to notice all the olm-men and frog-men in there.
So of course he shoots off towards them, and I'm thinking "Oh he's going to stand on the shore until they come out to fight". Boy was I wrong...
This dwarf JUMPS INTO THE WATER and finishes killing EVERY SINGLE CREATURE in the pool before he runs out of air. He then climbs back out of the water and goes off to have a light snack, as if this were an everyday thing. To be clear, he managed to slaughter and dismember 7 frog-men, 4 olm-men and one snake-man before he was out of breath, and then he climbed back out of the water. After that he was a 2 skill swimmer.
I gave him his own house on top of the mountain, with his own mountain tomb. He did eventually die...
At some point magma men had climbed out and terrorized my miners who were attempting to set up a magma channel. Urist McBadass notices this as he is chasing a werewolf and runs full-tilt for the two magma men. He kills one really quickly but somehow loses his axe. Can axes get stuck in people? Anyway this didn't deter Mr. McBadass, as he proceeded to WRESTLE A BEING MADE OF MAGMA. He lost both arms up to the elbow in the initial grab, and then (I assume, because he had no hands) bit the magma man to death before dying himself in the heat.
It was a sad day, and I entombed his artifact golden axe with him, along with his masterpiece steel armor. We locked the double golden doors and bricked up the entrance with his favorite stone, stibnite. I had been pitting the bodies of everything he killed into his tomb, starting when he achieved champion status, so there were quite a lot of bones in there.