In this scene, the group has just finished a battle against a surprisingly murderous old woman, and they're trying to figure out what's going on.
Martha is probably still alive, since she had a very large pool of healing last scene and nobody particularly wanted to kill her. She might have run away, but it's not likely, because she wants to get the group's help in rescuing her grandson.
If Martha is present, she first wants to tell everyone that she's very, very sorry, and that she never meant to do any of that, but the thing was evil and she couldn't help herself. She adds: "Nobody should touch that relic! I think it can do mind control stuff!", which is unhelpful since the player characters had already figured it out, but it's nice of her to try.
Martha explains what has happened to her: the museum had just gotten a new load of relics yesterday, and she was on the second floor of the museum getting them ready for display, and a bunch of people in hooded robes walked in, and one of them stabbed her, and then the relic she was holding turned ice cold, and somehow she killed all the hooded-robe people, and then she couldn't stop herself from running out into the square and killing a bunch of people there, and she was shouting "I'm sorry! Run away!" but some people didn't, and she's really really sorry.
Martha adds that her grandson, Timmy, was going to meet her in the museum for lunch. Um. Do the player characters think they could make sure he's okay? She can offer a reward, if they like. (Improvise an appropriate reward here. I usually offer a magical set of shoes of wall climbing.)
(If Martha isn't present, the group doesn't get this information or this quest, but the group still remembers that the woman holding the weapon was Martha Butler from the museum, and hopefully this will give them the clue that they should check what's happening at the museum.)
Anyone looking at the artifact can tell that it's magical, evil, and not destructible by normal means. Someone who makes an appropriate skill check can tell that this weapon hates all living creatures and is unusually good at killing them, that it steals the life force of those it kills and transfers the life force to the wielder, and that those it kills will quickly raise as undead under the weapon's control.
If someone manages communication with the artifact, perhaps if someone who speaks Draconic picks it up, they can learn that the artifact is confused and angry. The artifact was created by the Vs'stak Hegemony (the draconian tribe that worships the white head of Tiamat) and wants to destroy their enemies, ie, anyone that isn't them. The artifact wants to know where the Vs'stak Hegemony is right now. The artifact promises power to its bearer in exchange for cooperation in destroying its enemies, but the artifact is not a particularly good liar -- it's accustomed to getting what it wants through mind control, not persuasion.
There might be some tourist rubberneckers still here, if they haven't been killed and if nobody has made them leave. If so, they're happy to share information with the group (for example there is a sage present who can give the group the information about the artifact, from above), but it's a constant battle to keep them from touching the weapon. "Yes, I can tell it's powerful, but a strong-minded person such as myself should be able to handle it. Let me just get a closer look..." Any tourists who touch the weapon automatically fail their saves versus mind control, regardless of how "strong-minded" they've just claimed to be.
The buildings nearby have been slammed shut and are in the process of being nailed shut. Somebody saw zombies, and nobody wants the zombies to get in their building.
If there's a lull in the action, zombies attack. These zombies are controlled by the artifact, and the artifact has instructed them to retrieve it and give it to a living character who can wield it -- ideally a fighter-type who doesn't have good mental resistance, but any bearer is better than being stuck lying on the ground here. The artifact has killed quite a lot of people and has a lot of disposable zombies to attack the group with.
Ideally the group will come up with a plan for avoiding the zombies or killing them from far away, so that you don't have to run a combat with a group of zombies. If the group just wants to fight some zombies, that's fine too.
From this point, there's a small chance that the group will want to go to Dig Site Five, fifteen minutes' walk away. It's much more likely that they'll follow up on their only lead, and go to the museum.