I saw a ghost/echo/whatever the day of reset when they implemented the new delves. I had no idea why I was seeing a winged ghost of someone in the Earthcrawl delve and it confused me for a bit.
Definitely no one I knew. As far as I can tell, you’ll see tombstones and the ghosts randomly, though there does seem to be a slant to people you know on the tombstones, as I’ve seen guildies tombstones and we both have a laugh as they have found mine as well.
I’ve never seen a ghost of anyone I know, and it seems to be very random and uncommon to my experience. I think I’ve seen two in the same delve once in all my runs, but can see lots of tombstones.
The slayers can be weird, or at least I had one run in Sidestreet where I saw rares like Unknown, Slayer of X and a few “Unknowns” in the delve which seemed a bit of a bug as I knew what those mobs were.
They also seem to be recent, as in within a day of when I do the run, so those deaths aren’t old deaths, but recent deaths.