The well is full to the brim and goes straight down as far as she can see (which isn't all that far.) You recall seeing the well rise 35 feet from the lake to the bottom of the cottage, and another 10 or so feet up from the base of the pool, so it's evidently a long way down.

Titaani can hold her breath for one minute. So she could theoretically dive as deep as 60 feet – below the level of the lake itself – and still make it back up to the surface (assuming, of course, all goes as planned...)

Matilda stared after her gnomish friend with a worried look and her fists clenched.

Titaani takes this in, and fights back the feeling of panic rising in the back of her throat. She never was very fond of swimming, thanks to a naggingly irrational fear of raging water that she'd had since she was a child. 

At least this water is calm, she thinks, delicately lowering herself so her boot touches the water's surface. It's nothing like rapids or stormwater. It'll kind of be like a bath. A slimy, acidic and gross bath.

"Ben, please make sure the spear stays where it's supposed to be," she calls back up as she lowers herself down to her waist. "I'm going to dive."

With that, she draws in a deep breath, summons as much courage as she can, and dives. Perception: 16

Ben:

”It occurs to me, Titaani, if the clog is closer to lake level than our elevated position, we may be better able to deal with it by descending the stairs to the lake and investigating from outside the stone column.  If you can tell the obstruction is that far down, just come back up and we can go down the steps to the lake and take a look,” reasons Ben.  He holds the spear as she requests, but his main focus is still being ready to pull her out by the makeshift rope if she has issues.

Titaani jams the glowing spear into a nook in the top of the well's stone wall. Tying one end of Ben's makeshift rope around her waist, and the other end to the spear, she dives in.

The well continues downward as far as her stinging eyes can see through the cloudy water. So it is with a sudden shock that she hits an invisible membrane of some sort about ten feet down. There is an immediate sense of movement within the well.

Titaani: Make a Dex Saving throw.

The rest of you see nothing but a tethered gnome diving in, followed by a brief surge of overflowing water into Bavolrna's pool. 

Dexterity saving throw: 7

(Oh no)

Titaani takes 6 acid damage and is engulfed within a cylindrical plug of glutinous goo. She can't breathe and is restrained. The plug begins to surge upward, and more water overflows into Bavlorna's pool. 

Roll initiative, everyone! Ben is immediately adjacent to the 10'-high well exterior wall; the rest of you are 15' away (difficult terrain). You can decide for yourself (rolling an Intelligence check if you're unsure) if your character would sense trouble in this situation.

Initiative:
Gooey plug: -1

[Titaani is also unconscious. (gulp!)]

Ben:

Watching carefully, Ben jumps when the water starts  gushing up from the well.  “That’s not right…” he says worriedly.  Pulling himself up to the edge of the well and trying to grab the spear with the attached rope and pull in some slack, Ben looks down at his friend to and see if she is in distress.

OOC:  This is Ben’s reaction to seeing the water well up strangely, Some or none of it may happen depending on his rolls, but that’s his intent since he was watching carefully—with a a follow-up action to pull her out by the rope if she looks to be in trouble.

Initiative:  6

Athletics:  10

Matilda Initiative: 3

(Titaani alway seems to fall unconscious for these things - sorry guys!)

(( Horatio initiative 4 ))

Feeling the tense uneasiness build up in the others as tiny Titaani goes into the drain, a feeling of fear and danger overcome Horatio's instincts and he prepares to react. 

Initiative:

Ben: 18
Gooey plug: 11
Matilda: 10
Horatio: 4
Cad: 18
Titaani: (unconscious) 

Ben, sensing trouble, pulls Titaani's unconscious form free from the gelatinous mass with a mighty tug and reels her to the surface.

Matilda moved up to the edge of the pool and began splashing the water hoping to draw whatever it was out.

The thing surges up the well, striking out at Ben with a gooey pseudopod:

Pseudopod Attack vs Ben: 19   Hit: 11 acid damage.

With a torrent of overflowing water, the thing begins to crest over the well's walls. Ben, hanging on the top of the well holding Titaani under one arm, will need to retreat next turn or risk being engulfed by its transparent mass.

Cad, Matilda, and Horatio may act next. (The thing has half-cover from the well wall currently for everyone but Ben.)

Matilda ran in with her knife worried about her little friend, slashing at the pseudopod.

Attack: 18 Damage: 6

Action surge.

Attack: 17 Damage: 8

(( Horatio - completely unaware of the obvious and horrifyingly bad idea that is unarmed attacking an ooze - runs over and swings twice at it! ))

Unarmed Attack 1 = 10 to hit, 11 damage (in log)
Unarmed Attack 2 = 10 to hit, 5 damage (in log) 

"It's like a giant thing of snot! This is so disgusting! Forget that Hag, we need to lure this thing upstairs and make her deal with it!"

Matilda rushes forward through the pool and climbs the exterior of the well (move and dash), before successfully sinking her blade into the gelatinous mass (action surge; unable to make second attack.)

She easily carves off a hunk of it, which plops into the sloshing pool below. Unfortunately, there's quite a lot more to go.

Horatio follows Matilda through the pool and up the side of the well, but is unable to land a blow on the blob of goo yet. (Action spent on Dash.)

 Ben and Cad may act next!

Ben:

Easing down from the edge of the well, giving way before the nasty ooze to keep from getting enveloped but staying engaged so it doesn't get a free swipe at him as he moves, Ben sets Titaani down behind him away from the ooze, channeling healing energy into her as he does so (5 points healing from lay on hands).  His armor still sizzling from where the last pseudopod hit, Ben grips the spear all the more tightly and prepares to stab once he is sure Titaani is back on her feet behind him.