Pool of blood: gross and creepy. Do any of these people have the second key on them?

No, though the cleric does have a less fancy version of the same unholy symbol.

"This room is new; there was a cave here, but it wasn't this wide and it didn't have the lighting or decorations. Back then, there were two sets of branching tunnels; one of them lead to the other end of the maze, while the second was a dead end. Since I can't see the first one, I expect it's the one that was sealed, but presumably the expansion is taking place branching off the other one at some point, which means we need to go looking on the other side of that door."

Past the doorway is a long hallway. It heads downward, but via long straight stretches and then stairs rather than a steady slant. There are numerous doors off to the side of the main corridor.

She hopes the person they're looking for is at the end of the tunnel, because if he (or she) isn't they're going to have to go through all those rooms until they find them.

At the end of the tunnel is a set of prison cells, currently empty but with clear signs of recent habitation. Guarding the area are some red skinned humanoid demons, looking bored; there are what look like torture implements on racks along the wall, and one corner of the room is conspicuously free of cages.

All of that: fucked up! 

This room looks like a dead end but one of the demons might have the second key. She doesn't know how hardy they are and given the interior decor she's fine not pulling punches if the others don't know either.

"Cambions. They aren't all but immune to weak attacks like most demons but they're notably tougher than an untrained human, about on par with a veteran soldier. Try not to let them use their magic; it's not especially powerful but most of us aren't paladins so they'll sometimes succeed at controlling us if they try."

"Should I go in first so they waste the mind control on me, or would it target everyone at once?"

"Enthrall can hit multiple people, but it'll break as soon as they attack someone and you can interrupt it by stopping them from singing. We've got a lot of people here who can fight at ranged, so yeah, I think it makes sense if either you get in close with them while the rest of us hang back and shoot them. Maybe you and Seelah, if you think you'll want some help; she should also be pretty much immune to anything they throw at her."

"Alright." She and Seelah can line up and go in simultaneously on a hand signal. Demons: get punched.

And cut, and shot with arrows! The cambions are quite skilled with their scimitars, which is to say they are notably less dangerous than anything trying to fight Brenda back at the festival square, so she shouldn't have any serious problems dealing with them. One of them tries to command her to fall prone, though asking nicely probably would have been more effective, and about half a minute later none of them are in any condition to fight.

But do they have the macguffin of the day, is the question.

They do have keys on them, but none of them look like the first one or show up to detect magic; the same goes for any of the other sets in the room. The empty area of the wall does show up, though, which could be promising.

"Ooh, I think there's a secret door. Or something else is making the wall show up as magic but my guess is secret door." She starts poking around for hidden buttons/switches/etc.

There are four magical sources, each a button with a small painted rectangle above them in red, blue, green, and yellow.

"Great, I hate dealing with Baphomet cultists that think they're smart. It's usually not half as clever as they think it is, especially since they need their own people to be able to solve it, but it's always annoying trying to figure out what they're going for."

She stops, thinking for a moment.

"You know what, I bet it's either yellow red blue yellow, or that backwards; that's the order on the paintings back in the main hall and I suspect they use that as a reminder. They're certainly not for the ambiance."

"Is it likely to lock down or sound an alarm if we get it wrong, or can we just try both?"

"You should be able to try both, if it locked down every time it got one wrong they'd have to unlock it often enough to annoy them. It'll probably try shocking you if you get it wrong, since that won't hurt demons."

"Okay." Once she has her hands over the buttons, concealed from the others' view by the rest of her body, she deliberately adds rubber gloves to her ensemble over top of the magic ones. Yellow red blue yellow?

It tries to shock her, but in between her gloves and the magic gear she can barely feel it.

Yellow blue red yellow?

That'll do it! The wall slides apart to reveal a small room that apparently serves as storage for some kinds of valuables. In center stage is a golden longsword, but there are also a few items of jewelry with the same holy symbol as Seelah uses and some suits of armor.

From the moment she gets a glimpse of it, Seelah's eyes are on the sword.

"Is that Radiance? I know the orders said to find a better hiding spot for the paladin's sword, but I had no idea it'd be this one. I can't believe it wasn't destroyed!"

"Awesome! It's not showing up as magic; is it historically important?"

"Incredibly! It was used by the paladin Yaniel, a hero of the fourth crusade who ventured into the worldwound to rescue captured crusaders! She's an icon for a lot of us unaffiliated paladins since she wasn't part of any orders either."