You can pray at a gravestone, or possibly ring a bell at one, to cause any of a number of good and bad effects:

• Summon a ghost, which might be hostile or peaceful.
• Summon a hostile undead, flavored as it forcing its way up out of the grave. Usually a zombie.
• Polymorph you into a zombie or mummy of your own race.
• Put you to sleep, ignoring resistance.
• Create a swarm of hostile manes or lemures.
• The headstone falls on your foot, dealing some damage, less if wearing boots (“killed by a falling tombstone” if fatal).
• If the grave is a bones pile grave, the ghost of the player always appears. It will be peaceful if their alignment matched your own and hostile otherwise.
• Various cosmetic messages that do nothing. “The air suddenly turns cold.” “The engraving on the headstone shimmers for a moment.” “A shiver runs down your spine.”
• The dirt cracks open and spits out any items buried under it.
• A voice from nowhere whispers a random rumor, true or false.
• The epitaph on the grave changes. This does not flag the grave.
• Get an alignment bonus for honoring the dead (this is only if triggered by praying). “You feel that the one buried here is content.” Bonus might be larger if you’re lawful or a Priest, probably 1 otherwise. This should possibly happen before and independently of any other effects.
• Limited identify of 1 item.
• Type-identify of multiple un-type-identified items in the inventory, but the player doesn’t get to pick which. Might pick 0 items (flavored as the spirit not knowing anything new about what the hero is carrying).
• Enlightenment.
• Temporary telepathy / sleep resistance / infravision / other intrinsic.
• The gravestone rumbles and moves aside, revealing a branch staircase to a just-created-now one-level branch, a small one-off “crypt” floating branch that contains some undead, some boxes containing corpses, and some treasure.
• You “feel that the dead are restless”, aggravating monsters level-wide. All gravestones on the level become flagged against further attempts. This effect could be added to and independent of other ones, and if so, it’s a chance dependent on the number of already flagged gravestones on the level, which provides a nice way to balance it on levels that have massive amounts of gravestones.

After you get one effect, the gravestone is flagged so that it doesn’t happen again (or, more evilly, that any further attempts will only result in bad effects). Possibly shouldn’t work at all on graveyard levels, because they have too many graves to balance these effects.