ToME: the Tales of Maj'Eyal

I've lost two characters in a row to his multiple 200+ damage supercrits in one turn. Is he supposed to hit that hard?

Yes kinda
He is an optional boss guarding a powerful artifact

Could somebody give me a hint on how I actually get to the mummy boss?? I've gotten down to level 3 of the Elven Ruins, which is much larger than the other levels, but I've found no hint of the boss nor of stairs down. There is only one of those spots (like outside Angolwen) that just takes me directly to the surface. I've beaten the Master and had the staff stolen. Am I missing something obvious?

No, as you said the level is big, he could be hidden in a small alcove.
He is also invisible, but you'll probably notice him anyway when you start dying very fast

Does this boss show up in the quest log? And if not, is the artifact he guards a certain edged weapon that begins with 'R'?...

Yup

Gotcha...already nailed him... Guess I need to start paying more attention when I play!