REF: Spectres

I haven't done a Random Encounter Fun™ posting in quite a while, but –in keeping with the Halloween vibe– I did roll up a semi-random encounter* to try and cobble together into a (hopefully) fun little set piece.

*I did decide to use the Undead Sub-table, but I randomly rolled the location and type of undead, as well as the treasure, hit points, and number appearing.

I rolled "City" as the location and a total of four (4!) spectres. Yikes!

I decided it made sense that three of the spectres were underlings, drained by the "boss" and under his sway (as per the description in Cook). Hit points came out to 36 for the boss and 24 each for the lesser spectres ("lesters?")

When I rolled treasure, I was bemused to get a result of 4,000 silver pieces and 5 gems. Why would an undead tolerate the presence of so much silver? But then I read in the description the spectres "... have no solid bodies, and can only be harmed by magic weapons: silver weapons have no effect." So no problem there, other than why an incorporeal undead wants cash at all, but we're getting to that.

While it doesn't specify this in BX, the Rules Cyclopedia gives this creature an average INT of 8. This isn't a genius, but it is sentient.  Also, it's possible our little academy of apparitions (yes, apparently that's the correct collective) is smarter than the average spectre. Let's stick with 8 for now, shall we?

So we've got four of these terrifying but not so bright undead spirits –with a modest pile of loot– hanging out somewhere IN A CITY. What gives?

It seems to me that in a city of any size, the local temples would have destroyed these things pretty quickly. So it one of two scenarios seems the most likely:

1. The "infestation" is recent.
2. They are in a relatively isolated location.

As this was rolled up as a random encounter, not a predetermined part of an adventure, I don't want to overthink the setup. So here goes:

A few nights ago, three ruffians knifed a merchant and took his money: a coffer of silver coins. Pursued by the watch, they jumped the fence into the local graveyard. They hid in an old tomb. While waiting for the coast to be clear, the thieves spied a gem-encrusted urn on the altar. Uninterested in the contents, they smashed the vessel and bent to scoop up the precious stones. 

The urn was magical and trapped an evil spirit. The family had built the tomb when the wicked patriarch had died because it was expected of them and they wished his evil to remain a secret, but they knew his foul necromancies might allow him to return from death in some form or another, so they cremated him and had the vessel enchanted to hold his spirit. 

Free of its prison, the spectre quickly slew the thieves and they soon rose as his spectral slaves. Their bodies, and the treasure, lie on the cold stone of the tomb's floor.  The family died out generations ago, and no one ever visited the grave while they still lived, so it is largely forgotten. The spectres have no master plan, and they shun the daylight, but if anyone were to stray too near after dark, or enter the tomb, they will gladly feed off his life force.

Why are the PCs in or near the tomb? Maybe they are searching for the entrance to the catacombs, maybe they are chasing someone? You tell me!