I just started making my catacombs... a winding passage of graves, statues, armor stands, weapon racks, and random engravings on unmined pillars. I'm smoothing all the walls, but not the floors, because I like how it looks. The only finishing touch will be to dump out all the stone in there... which will... take time. It's gotten quite big, and has 83 coffins in it. Arriving nobles will get their own custom and decorated chambers carved out in the same area.
I have a hall near the main-drag of my fort with tombs for my starting 7... I was careless, and I've lost one already in year 4 to a goblin ambush. She saw a thief, panicked and ran outside into the waiting bow-gobbers. She and her husband were my fort's best carpenters, with her being near-legendary and him apprenticing under her tutelage. I sigh a little every time I order up a new batch of barrels, it must be hard on him, since that's likely how they met.
I hadn't thought of entombing anyone who hits legendary, I need to get on that. After all, I'm planning on pampering my nobles, so it's only just that I give just deserts to those whose labor makes it all possible.
Also, Hellzon, thanks for the ore vein idea, I think those will become Auxilliary catacombs. I hit a crap ton of Lignite and Bituminous coal in year 2, and have been wondering what to do about the massive scars they've left in my fort.