Make a Religion check, anyone.

Religion: 5, 21 if with advantage.

"I'm not an expert on these things, but I'll give my noggin' a crack at it." mutters Eima, talking out loud as she thinks about things, hoping that perhaps she'll trigger a chain of thought in her companion.
[[ help Tezzy  with the religion check. ]]

"Pews. Creepy lights. Weird symbolic statue. I'd say this is the cultists' place." she says. "But that begs a question. Where are the cultists?"

The statuesque figure is almost assuredly the one and only, the formidably, Vecna.

Tymm looks upon the statue. “And who is this handsome devil?”

"Seems a place where they come to worship Vecna, deity of secrets, undeath, and dark magic. This, my friend," says Tez pointing at the statue, "would be the likeness of the Whispered One." His glance wanders carefully over the large room. "We must practice caution, there are sure to be traps with curses and whatnot, and probably more undead. Also keep a lookout for secret passages and compartments." 'Has there be recent activity, here?' he wonders to himself (Perception (+2)/Survival (+0): 1 (+1 from Guidance)).

Sky approaches the figure. "Impressive"

"You are correct, there may be traps and such. I'll keep any eye out. "

The area does seem well used, just suspiciously quiet. There don't appear to be any traps in the area.

"Only madmen would be cultists for such a thing." murmurs Eima as Tezzy explains. She looks at the altar, "Two paths. Left and right." She gazes at the statue and pews, "I'd not like to be here when they all come gathering. Do ye have a preference of which way to go? I'd reckon whichever way the hand is not pointing."

[[ IIRC there were two passages, right? The discord image is no longer showing up on my end. ]]

(Yes, left and right)

Tymm pipes up. “I usually favor going right. I hit better with my right hand so I mostly just follow that and so far I haven’t died…”

"All right, right, sounds like a practical choice. I'm right behind you, then," jokes Tez and follows Tymm if he takes the cue.

Crooked shelves filled with books and scrolls stand against the walls. In the center of the room is a square table covered in scribbled notes and ink-stained parchment.

Already forgotten his own warning, Tez begins rummaging through the shelves, cursory inspecting scrolls and books alike (Perception: 11, including 2 from Guidance). 

Eima smiles at Tymmy's reasoning. Simple but effective. "Can't argue with that." she says, heading down with the rest.

As Tezzy rummages through the shelves, Eima searches for hidden passages. Poking at the walls with her thieves' tools and daggers for hidden triggers and other possible modes of accessing further regions.

Investigation: 8+5 = 13

Quote from cyreon >>

Already forgotten his own warning, Tez begins rummaging through the shelves, cursory inspecting scrolls and books alike (Perception: 11, including 2 from Guidance). 

 Tez finds a small ebony box among the books on the shelf. It appears to be locked.

Quote from VitusW >>

Eima smiles at Tymmy's reasoning. Simple but effective. "Can't argue with that." she says, heading down with the rest.

As Tezzy rummages through the shelves, Eima searches for hidden passages. Poking at the walls with her thieves' tools and daggers for hidden triggers and other possible modes of accessing further regions.

Investigation: 8+5 = 13

 Other than a hallway to the south, there doesn't appear to be any other exits from this particular room.

"I ain't finding nothing in the walls. Best I can tell there's no further passages." says Eima. Turning to Tezzy she asks, "Ye had any better luck?"

„Nothing in way of a passage,“ Tez admits to Eima. „But look at that!“ he holds up the ebony box. „It doesn’t open. I guess that’s a good sign.“ He offers the box to Eima, should she want to take a closer look for herself.

"A box that doesn't open?" says Eima as she takes out her thieves tools with delight, "Let's see if we can fix that. Mind giving a bit o' guidance?"

She goes to work on the box.

(believes Thieves' Tools is DEX-based.)
Thieves' Tools: 9+5+1 (if Tezzy guives guidance) = 15

Not only will Tez provide Guidance, but also help Eima, if she'd let him, and, of course, if the need arises. (Tez also has proficiency with thieves' tools, so I guess he can help efficiently if it makes sense mechanically. He could hold still the box or a pick.)