Matheris whispers signals he agrees.  You coordinate to get the good yuan-ti and bring them.  They will join once you make your move to distract the others.

Matheris signals they should focus on the Yuan-ti Malison.  Once those are down the others that still live will likely run.  They will go tell the others and they will either come for a final fight or flee.  21

This was a bit more invovled signal, but he thinks it was easy enough to get across.

He signals for Faith and Karea to work together and he and Lena will get the other one.

With everyone ready you get surprise the first round.

Roll initiative:

OOC that was Matheris' stealth roll FYI..

initiative for Matheris 19

initiative for Karea 19

initiative for Lena 19

I'll post Faiths actual initiative later...For the surprise rnd it's not as critical for the roll.  Everyone on the good side can go with advantage since they are surprised and they dont' see us.

Lena 25

Faith 20

Yuan-ti Pureblood A 18

Yuan-ti Malison B 17

Yuan-ti Pureblood D 16

Karea 15

Yuan-ti Pureblood C 14

Matheris 13

Yuan-ti Pureblood B 12

Yuan-ti Malison A 8

Lena, Faith, Karea and Matheris are up:  Go..

Faith and Karea focus on Malison A.

Lena and Matheris will focus on Malison B that also has a Yuan-ti Pureblood too.

Since the Malison are positioned one in the northeast pool and one in the southwest pool, we set ourselves up to be 30 ft away from our targets ready to go.  Malison B is in the southwest and Malison A is in the northeast.  1 Yuan-ti pureblood (A) and 1 Yuan-ti Malison (B) is at the southwestern pool (5), going northeast next pool (4) has Yuan-ti Pureblood (B), continuing Northeast Pool (3) has Yuan-ti Pureblood (C), continuing Northeast Pool (2) has Yuan-ti Pureblood (C), continuing Northeast Pool (1) has Yuan-ti Malison (A).

The pools are stretched over 350' from tip to tip so we'll be fighting a bit separated and on round 1 the Good Yuan-ti will move to distract the middle Yuan-ti Purebloods but stay out of reach.

I will delay posting for Lena and Karea for 24 hours or so in hopes they can post for their characters.  In 30 hrs I'll post for them though.

Initiative: 22

Faith focuses on the Yuan-Ti Malison that Matheris has indicated should be their portion. This has to be hard, based on what the good Yuan-Ti told them about the danger to the Grippli and these ponds. 

Unless…

Faith focuses his wish upon his foe and casts Suggestion. (Save: Wis 15). 

“Tell your evil kin to flee, then flee with them, and don’t stop fleeing until you’re eight hours from us. We have killed your leaders in the temple, and our allies come behind us to kill all they see.”

Faith cast his spell on the Yuan-ti Milason A.  The creature has advantage on the Wis roll +1 and rolls an 8 and a 1 for a 9.  Fails miserably.

Karea hears Faiths comments and is smart enough to know what has happened but is unsure that it has worked.  Karea whispers to faith,   "Faith, what is the meaning of this?"

Matheris

In Hybrid form as they enter the temple. 45 min left.

Movement: Move 25 ft to be behind the Yuan-ti Malison B

Rapier of Warning Attack: Yuan-ti Malison B: Attack: 15 Damage: 6 (Slashing) + 1 (Sneak slash) + 8 (Green Flame) + 2 (Lighting) + 6 (Necrotic) = 23

Damage Second target Yuan-ti Yuan-ti Pureblood A: 9 (Green Flame damage)

Bonus Action:  Hex on Yuan-ti Malison B; Disadvantage on Wis saves.

Reaction: Opportunity attack

Matheris nearly kills the Malison B on the first attack both Lighting, necrotic and fire assail the creature along with the blade.  A sneak attack hits as well  Malison B staggers hissing loudly from the brutality of the attack.

The flame leaps from the Malison B to the Yuan-ti Purblood A as it screams from the fire.  

Lena is up:

MOVE: Lena advances up to 30 ft to be 5 ft from Yuan-ti Malison B and  Yuan-Ti Pureblood A: (spending 30 feet of movement; first diagonal costs 5 ft. of movement, second costs 10 ft., third costs 5 ft., etc.)
BONUS: Two Weapon Figting: Attack Yuan-ti Pureblood A: 18 Damage: 18 (Peircing); Attack: 18 Damage: 16 (Peircing)

ACTION: Attack Yuan-ti Malison B: Attack: 25 Damage: 9 (Peircing); Attack same target if still alive or Yuan-ti Pureblood A: Attack: 25 Damage: 6 (Peircing)

Dread Ambusher attack on First attack only; Attack: 18 Damage: 7 (Peircing) + 4 (Peircing) = 11

Reaction:  Opportunity attacks or any available.

Karea thinks she knows what Faith is up to and holds her action:  If the Yuan-ti don't leave she will attack with severity.

The Malison does in fact marshals the other Yuan-ti under the suggestion of Faith's command...Some of the othe Yuan-ti balk for a moment, but get the impression they have already lost or they would not gathered up in this manner.  You clear out the Pond Mother's home and then move to the other homes and gather the rest of the Evil Yuan-ti and escort them away from the island and they travel as far as they can away.  The Grippi are saved.  The Good Yuan-ti can continue their work on the temple under a more closely guarded process by the Grippi.  Just to be sure.  

The Pond mother, "Thank you brave adventurers...You have saved us from destruction and slavery.  We are in your debt."

You may keep the [Magic Item]Chime of Opening[/Magic Item]  5 uses left; The [Magic Item]Lantern of Revealing[/Magic Item]  2 hrs of oil;  I have opened the sarcophagus and you each may take a [Magic Item]Serpent Scale Armor[/Magic Item] and [Magic Item]Serpent's Fang[/Magic Item] with you.

We have prepared a ceremony in your honor at midnight tonight.  If you will participate it would be an honor for us to do this for you.

XP awarded per player: 10,115 xp per player  That all the creatures and the bonus for saving the eggs and treasure.

I believe that will level most of you once.  Maybe twice for Faith. 

If you participate in the ceremony....I will assume you do....In the meantime, if the characters prevented one or more eggs in the brood pools from being killed, the returning grippli are so ecstatic that Pond Mother offers to perform a ceremony for the party’s benefit, called the ritual of friend marking. If they agree, they are brought to the pool in the center of Pond Mother’s home at midnight. The grippli, croaking sonorously, surround the characters while they stand in the warm water and Pond Mother pours the contents of a jar into the pool. The liquid swirls, and motes of light begin rising up around the characters. Any party members who have second thoughts at this point can avoid the effect by succeeding on a DC 15 Wisdom saving throw. Party members who don’t resist, or who fail their saves, fall into a trance until dawn—when they come to their senses, completely rested and healed. All party members who completed the ritual can speak and understand the Grippli language from now on.

Each player gets 15 x 100 gp pearls that the Grippi get from the oysters they cultivate as a thank you.

The Boat is waiting for you and it's not as much of a challenge to get back to the library now that you've done it once and you have the skills to make it a breeze.

You take a long rest while on the trip back you train your skills and get acquainted with you new found strengths. 

The pond Mother speaks to you in Grippi, "Your weapon's can be adjusted to your personal taste.  Dagger, Shortsword, Rapier or a Ring that will give bonus' to unarmed fighting and claws."  OOC - this will require your characters to use Homebrew items so you'd have to select homebrew in character options. PM me if you have any questions.

The trip takes 36 hours and leaving at 7 am puts you back to the library and 7 pm just at dusk.

"You are greeted by Pelk on return.  Welcome adventurers...You were successful I hope.  Please take time to rest.  Rooms have been prepared for you.  You can get food and refreshments to your liking.  We will make sure it is paid for you should you wish to rest here before returning to Phandelver.  You could remain here if you choose as well or leave right away for Phandelver.  The choice is yours."

I think I am NPCing both Lena and Karea while Playing Matheris.  This will mainly be Faith's choice unless the others chime in over the next 24-36 hours ish.

Actions:  

Returned to the library, and rested Faith meditates on the choices. The stress of the Grippli excursion weighs on him. Even a night in the keep might be appreciated, and with what he had learnt from the battles and the evil magics he witnessed from the Yuan-Ti he feels ready to take a second crack at the scrolls he once earned in the forest of wonders. Perhaps this time he might master a ritual of safety, for the next time he finds himself whisked into a new adventure. 

Soon, however, Phandalin will call. There was too much left undone. Mysteries unsolved. 

Lena looks at each of her companions and turns to Pelk, "Yer gone stole us away from our business with yer trapped book. As happy as I am t' help yer cause, I made promises t' Gundren to aid him. This diversion has me breakin' me vows, which ain't right fer any god-fearin' soul. Now, if yer be needin' help in the future, I'd suggest yer ask first an do yer kidnappery last..."

Pelk nods, "It was not our doing necessarily my lady.  You see there are teleportation rings scattered about Faerun.  The book activated a ring as it had the spell sigil associated to the ring here."

"It sounds like a rest here and then we can take you to the ring again to teleport you back to the castle. Enjoy your evening and a good long rest.  If you wish to return I will give you a necklace with the sigil to teleport you back here.  The necklace will work 1/week and activate a teleportation ring linked to here."

Lena eyes Pelk suspiciously, "Yer were most fortunate then. That such capable folk set off the book in a time of need. Still, some coin fer our time, an a speedy return be welcome. This amulet, yer be sure that we can choose if we want to be usin' it?"

She accepts the amulet with a certain amount of trepidation, eyeing thoroughly it before placing it into a pouch.

Pelk looks slightly intrigued and pulls out 5 pouches and hands one to each of you 100 gp, "That should help ease your concerns for being helpful to the Candlekeep's library.  Unless you have further need of me, I will retire for the night.  If you should need me one of the other Avowed can get me."

The next morning you all meet in the Hearth/Tavern to discuss you day's events.  Pelk meets you there and says, "When you are ready and finished Breakfast should you want some I can take you to the room you arrived in.  You simple activate the necklaces using the command "Potelert" While holding the necklace's stone with the Candlekeep's sigil in your hands at this ring or the castles ring near Phandelver.  Again, Thank you for your help.  Good journey to you."

"Please, no," says Faith taking the pouch of gold. "You've done too much already, and helping the Grippli was its own reward." He tucks it securely, deep in his bag of holding. 

Matheris and Karea chime in, "I'm ready to get back to Phandelver too.  Seem we have some unfinished business there.  Shall we get going?"

Karea and Matheris add the necklace to their gear too.

“I am ready and willing,” says Faith.