Morwena continues "I'm not aware of any sort of treasure Sylvene may have left behind, but you know I do enjoy a good puzzle.  Might I see this key you mentioned? Every key has a lock and generally complement one another.  Perhaps together we can deduce what this key was meant to unlock."

<who actually has the key now? It was whoever opened the book the fou d it and any of you who could read or be read to could have used it, so unless I hear otherwise, I think it was Marlyth that found the book and had the key. >

(Let's see) Sleight of Hand 17

(If Marlyth wouldn't notice, Weneon could possibly have "handled" the key, we can play that out if you want)

Weneon smiles "This reminds me of cousin Durfoot's gully dwarf inn. Though the salt there is from sweat and not sea. Have you ever been to Krynn? You seem like one of those that might follow that Dantilis fellow. Odd magics with that lot."

She moves a rook from one end of the board to the other and then flips the piece on its head "Now it can move on top of other pieces yes?"

Attempting to poison a former Red-Wizard Mentor (Morwena Veilmist) may seem like a striking idea but it has proved to be a terrific one. Thus, Marlyth takes a mental note to master the art of trickery with the aid of a fellow Rogue, it doesn't matter if it's Tabaqui or Weneon. 

Sitting barefooted and cross-legged on the sandy beach of the magical demi-plane, Marlyth decides to cast Conjure Animals. Two fey creatures appear to occupy unoccupied spaces when she whispers to the spirit of the nature and uses her Totem to summon them temporarily within the range of 60 feet. The creatures take the form of beasts that are inhabitant of the Coastal Area (Giant Seagull) and they wait for her command words.

She commands one of the creatures: "Scout the place and find out the owner of these footsteps". She sends instruction for the other bird to examine the flora and fauna of the island to search for herbal products.

First Seagull: Perception 18

Second Seagull: Investigation 11

(@Wile_E_Coyote, Sir Angus found the key and inspected it since Marlyth was too busy with the contents of the Book to pay any attention. Marlyth had the book in her inventory with some personal items.)

Angus will produce the key, but keep a firm grip on it.  "This is what we found.  Do you know what it might unlock?"

She rises as Angus produces the key.  "Hmm.  May I see it please, she holds out an open hand.  It looks familiar.  I think I've seen something in thus house that looks sort of similar. Now where was it... I think maybe in the basement." She says as she looks to the next room to the south.

Angus can just hand her the key or not. I'm going to assume he does, but if he doesn't want to, then...

The seagulls see the footsteps walking toward the corners of the island (2 northwest and 1 southeast) but no other people besides yourselves and Morwena.  It only takes a aboout 20 minutes to cover the 6 mile roundtrip, but it hasn't been that long yet.

Angus does not hand it to her.

Wisdom save:14

Perception vs Sleight of hand: 11  

(Damn, crap rolls)

Well then, Angus heads to the cellar.

(She casts suggestion, invisibility and performs sleight of hand all in one turn before anyone can react?)

Tabaqui will Search to attempt to locate and follow the wizard.

Investigation: 28

Perception: 23  

If located, can I get off an attack on her?

<It's not combat so I'm not tracking every character's actions in 6 second increments, but each single target action/spell was (potentially)  immediately contested by someone.  I could have made her not turn invisible, but it seems like what she would do.  Since she didn't have to snatch the key at all, she could cast invisibility at her leisure, perhaps after ducking just into the next room as if leading the way to the cellar.  Mechanically she had cast 1 spell in the great hall(feels like another library) contested by the target and another in the next room.  I just jam posted so we didn't have a tedious back and forth between dm and 1 player for a whole day. Also, it's better for you if she did cast multiple spells if your inclined ro fight her, that's fewer slots left for her to use in combat.>

In the southeast corner of the house is the dining room, a square, hardwood table dominates this room, surrounded by enough high-backed wooden chairs to seat eight. The room’s two sideboards look like they’re made of driftwood. A painting on the southern wall depicts a green, grassy barrow with two tall steles in front, etched with spirals.

This room is in good shape and largely undisturbed. The sideboards contain a complete set of silverware worth 125 gp.

The painting is a wide, framed landscape called Wizard’s Barrow, by a once-famed Waterdhavian artist called Saraque Skyhill, whose work Sylvene admired. It is worth 550 gp.

<Invisible creatures are Heavily obscured,  meaning those searching them are effectively under the blinded condition by which all sight based ability checks automatically fail.  However...>

just to your left is a door.  Tabaqui thinks he hears something in the cellar.

Actually you have to pass through the kitchen to get to the stairs. (I though they were on the other side of that wall. My mistake, so she actually walked from room 2 through room 3 before vanishing in room 4.  Sorry about that.)

Kitchen. Room 4

Loose doors on the south wall let in crisp air. Empty barrels and neatly folded burlap sacks say this kitchen has been neatly emptied. Smoke stains on the plaster show it has seen a lot of use. Stairs descend into darkness to the north.

The fireplace boasts a large stew pot and roasting spit. The shelves and cupboards here are bare. A collection of common knives and spoons slowly rusts in a small chest here.

The stairs lead down to the cellar

Marlyth assumes that these footsteps might belong to another wizard who might have teleported here with Morwena Veilmist. She seems curious about the marks and attempts to examine the exterior of the house as well as the place adjacent to the building so that she might be able to pick up more clues. As a result, she gradually moves to the southern side of the site in order to search for an alternative entryway (she possibly stands outside the kitchen).

Intelligence (Investigation): 23 (Proficiency bonus doubled: Natural Explorer feature)

So when Tabaqui saw her duck out of the room, he immediately followed her.  If he catches up to her in room 3, he will attack, if not he will follow to room 4. When she turned invisible, he would have used the perception roll (which includes hearing, not only sight) and tried to get off an attack (at disadvantage, which might be countered by his assassinate feature).  [Will you allow this?  If not, then Tabaqui will immediately proceed down the stairs to the cellar, and there make another perception check.]

Perception: 13

Weneon darts after Tabaqui, she will try to locate their quarry as well

Perception 23

As his bonus action Calner will initiate Telepathic Speech with her and ask directly to her mind, "Where are you going in such a hurry?"

After she goes invisible Calner will use his action to delve deeper into her thoughts via his ongoing Detect Thoughts spell.

Morwena needs to make a WIS save vs. DC 16 to end the spell. If she fails, I gain insight into her reasoning (if any), her emotional state, and something that looms large in her mind (such as something she worries over, loves, or hates).

The cellar.

Bare stone and wooden rafters make up this low, six-foot-tall cellar. Dry sand is scattered throughout a room that is almost parched—despite the nearby seashore. Perhaps this windowless cellar was magically made to keep the kegs, barrels, and sacks here from moldering.

There is a stone is set in the center of the ceiling. It is a dark, jagged stone with veins of gold, all etched with arcane runes and sigils, very much out of place here. It’s about the size of a loaf of bread and weighs about 10 pounds.

Three bottles of fortified wine from the Sword Coast, never opened, still sit amid straw in a crate in the southeast corner of the cellar. Each bottle is now worth 70 gp.

Morwena Wis save 11

Calner hears Morwenas thoughts as she struggles with the betrayal she's faced.  The betrayal she committed and what she must do now that she's been forsaken.

{When he looks at the Realms, Tyreus only sees a place to be conquered or plundered out of spite.

Morwena tried (and failed) to convince him the Realms could make him powerful—that he could carve out a part of the Sword Coast for them to rule together—and that’s when she knew Tyreus would not share power.

Tyreus is an angry and sometimes petulant person. He thinks if he gathers enough power, he can make a place for himself in the world where he can be strong and, finally, find happiness.}

<See, if you talked to the last wizard you'd already know who Tyreus is.  You got his name back there, but that's about it.  Technically, someone has already seen him, wearing a green cloak and holding a white book.>

Marlyrh strolls outside the house through the solarium

Long and empty tables run the length of this room, along the walls and down the center, stained where pots and planters once sat. Now they’re all stacked beneath the tabletops on a floor still stained with good dirt. Sylvene never got around to accomplishing as much gardening as she intended. Nothing grows in this room anymore.

A golden disc, about the size of a platter and made to resemble a stylized sun, hangs on a wall in this room. It is worth 250 gp.

Outside, she finds a garden.

Overgrown to the point of choking on itself, this garden is dotted here and there with signs marking the original plots of herbs and vegetables, many of which clearly no longer grow here. The air smells like sweet basil. Lively herbs have overtaken much of the garden, but some carrots and turnips still grow here, too

Random roll: 6

Angus amiably walks into the cellar as he was instructed and looks for something the key could open.  He examines the stone in the ceiling... are there any keyholes?

Investigation 16

 (So does Tabaqui have any idea where the wizard is?)

"We would like our key back," he says.

Marlyth orders the first Seagull to guard the entrance when she examines the contents of the garden. She instructs the other bird to pluck some fresh carrots and turnips while she attempts to hear if there's some kind of movement inside the house (Passive Perception 15). Finally, she attempts to remove the Golden Disk from the wall at the same time crying out:

"Hello! Who's there? Mal? I saw your footsteps outside and don't try to escape from the person who saved you. That's uncivilized."