Forum:What have you used just once?
OK, I'm a simple guy, play Neutral Barbarians exclusively, and don't use/need many things the game provides, except as polypile material.
But one day some version of a Lich was created in one of the lower levels of the Gnomish Mines, with the biggest problem being that it constantly created new high level monsters, like the Titan on one Medusa level, some monsters being Black Dragons, and the result was a bones level I kept running into that I couldn't get past, either eventually getting killed by not retreating up the stairs fast enough or getting disintegrated. Or maybe there was a ghost on the stairs that I couldn't kill fast enough to use them.
I started only going down as far an Minetown, then retreating back up to the main branch, trying to build a character good enough to survive the <whatever>Lich, then the gang he'd created.
Couldn't get reflection and the MC I needed in the time I was willing to wait for a long time.
Finally I used a scroll of Confuse Monster, for the first and only time.
Killed the <whatever>Lich, then engaged the Black Dragons and killed them.
I was expecting the mother of all bones levels, but unfortunately the Black Dragons must have disintegrated much of it trying to kill the many versions of me.
This very-high-level-Lich showing up earlier than I'd expect has made me a fan of a ring I never thought I'd have much of a use for, but that is another story.