I had an edventuring companion who killed more than a dozen hydra along side me die by drowning in a river while I was sleeping.
I found him lying on the shoreline, which confused me even more. After liberating any of his gear that was better than mine I set about getting revenge for this worthy ally.
I didn't know what to reek vengance on however, as his stupidity was the true culprit, and as long as there are dwarves, and walls to build themselves on the wrong side of during an invasion, stupidity will never go away.
My second option was the river.
I retired the adventurer in the nearest town and started a fortress at what I assumed to be the srouce of the river. Then abandoned it, sent my adventurer to the newly created fort to retire, then reclaimed the fortress.
With my new legendary and vindictive overlord to guide my hand, I knew justice would be swift at hand.
I dug as deep as I could, as fast as I could. Neglecting everything but the task at hand, and the parts needed to accomplish it. Many dwarves died in needlessly painful ways that didn't need to happen for this goal, but by Armok it was achieved.
I dug until i hit magma, pumped the magma back to the surface and strangled the river to death with molten !!JUSTICE!!. Then as an added insult had my surviving miners carve and expletive aimed at the once mighty river into the obsidian and mud that it now consisted of.
What completely necessary ends have you gone to that once accomplished you couldn't help but giggle madly at, then worry about your sanity?