SILLY NITPICKS:

Comment: Is the ice-white unicorn, kelpie scales, and token of a water beast's might something that's needed every solstice? Is there someone whose regular job is to get these things who is somehow busy or unable to do it?
Suggestion: Maybe this festival is more important and less often than once a summer? Perhaps the famous silversmith used to be a powerful magical adventurer but has since retired, and they used to go get the stuff? Alternatively perhaps this famous silversmith is wandering and has attached the need for these special items to the town's regular solstice festival?
Wacky suggestion: This town is EXPLICITLY a retirement home / colony of former tier 4 adventurers. The tavernkeeper used to be a high level barbarian who is in a civil union with a faerie king/queen, the *only farmer in town* is an archdruid who felt too bad for their animal friends getting hurt in dungeons, etc. The town council has a bunch of old fart wizards and sorcerers constantly bickering and having cantrip slapfights, that sort of thing. The town hall is actually built out of gold coins and the actual currency is low-level magic items with low weight etc.

SERIOUS COMMENTARY:

OK reading further into the Adventure Background, is Aine assuming the identity of the town's silversmith just for this year or has she been the silversmith since time immemorial? Also, I'm imagining that in the course of this game, Aine and Greine actively tune the difficulty so that for the group of level 17 PCs there's appropriate challenge, but for the youth of the town it's a little more possible to complete.

It might be worth having some people mention that this ice-white unicorn is SPECIAL somehow. Seems different, doesn't act quite right, does something somewhat out of character for unicorns perhaps. Unicorns are rare in the D&D-verse but not exactly mythical creatures like they are in our universe, which makes general knowledge about their lifecycle a little more widespread.

Does anybody die when trying this competition? Are the entrants typically youngsters from the townsfolk seeking to enter adulthood, or is this something where there's going to be hardened world-famous heroes from all over flocking to Giant's Bank? Ah, I see in the section where Aine is introduced the brass tacks (lol) is full of other adventurers. I think the tavern should be too. Lots of people swapping bullshit tall tales and various nonsense rumors about what they think the Water Beast might be, for example.

I might humbly suggest not going with Loch Aer and instead Lake Aer to help downplay the origin story, but that's a SOFT suggestion at best.

I also think "token of water beast's might" is a bit vague. I'd go for simply stating "a water beast's tooth" but even then... imagine if there's a merfolk PC and they offer to have the barbarian knock a tooth out and the cleric fix it up. Genasi and Merfolk both might misinterpret that and just offer some items as literal water beast's offering literal tokens, possibly even of literal might. You know if this is a regular thing, there might be really specific information about what the water beast is, how big it is, etc from multiple people who had completed the task and given their story to the townsfolk in years gone by. After reading further, Plessie is going to harass the party no matter what, so it's logical that Plessie attacks ALL shipping. The townsfolk might speak about it as basically guaranteed: "If you travel to the isle of Lisay, the Sea Beast will find you and make itself known. You'll know it when you see it." perhaps. Hell, con artists might be selling cheap Water Beast lures and repellants!

Aine's prize being 'worth it' is worrisome. You might make it a package deal - "I'll give you a token of my favor - it'll be worth it - as well as one magical item of your choice" from this carefully curated list perhaps.

The Plessie fight! Are there any telltale signs of a huge aquatic predator being in the area? Is Plessie's arrival presaged by small fish, then large fish, then SHARKS all frantically breaching the surface, all in the direction Plessie is approaching from? Does it go eerily quiet? Is it calm all around but with sudden strange waves coming with ever increasing frequency from one side as Plessie swims towards the ship? What happens if the party *doesn't use a ship* and just casts waterbreathing and walks underwater or swims or teleports?

Arriving at Lisay: WHY does the path go only to Loch Aer? Is there a sign up? I mean, it's what the party is here to do, but how do they know how to get there? Is there a high tree to climb/high cliff they can spot that would give them a commanding view of the island and good vision on the general layout? Also - what color are these unicorns? I guarantee the party will ask if any are "ice-white". You might have three or four of them be a bright white, to help introduce the idea that this 'ice-white' creature is something special.

Firgmar's Field: Is there a sign up saying that this is Firgmar's Field? Are the townsfolk likely to describe the layout of Lisay as consisting of Loch Aer and Firgmar's Field as important places to investigate? Also, the Ice-White Unicorn should SPARKLE and possibly be BLINDING to look at. Make it really explicit: This is weird, and THIS is the iciest of whites. I might even recommend invoking temperature to help associate the whiteness of this unicorn with ice by saying an unseasonably chill breeze momentarily wafts over the sleeping unicorn to the party. ALTERNATIVELY, what if its horn is literally translucent blue, magical ice? Can animal handling, or a Druid speaking Sylvan, resolve the Ice White Unicorn encounter any differently? What about when paladins use divine sense, does this unicorn-that's-actually-a-goddess burn out their divine sight for a few hours? What about someone using Tongues or Comprehend Languages to try and negotiate? Or Force Cage / Hold Monster to just pin it in place and have the Fighter go put it in a sack? Basically - what's in this encounter for parties who do not have lots of access to Stealth checks or Survival checks? How does this encounter unfold when high end characters brute-force it with super high-end magic and character abilities?

To backtrack for a second, much of my questions about the Ice White Unicorn encounter apply to the Kelpies. Can the party barter for some kelpie scales? Befriend them honestly or magically temporarily and just get given these scales? Are they pliable with gifts or boozahol? Can they be impressed by feats of strength or intimidated by force of will?

THE PLESSIE STATBLOCK:

Can Plessie be surprised? Your local Rogue Assassin subclass is *interested* in knowing this.

Plessie should vanish or depart at half health. It's gotta be there for the next ritual after all!

Plessie's bite should be magical piercing damage.

Plessie should be encrusted in various arrows and swords and magical items from tangling with adventurers year over year. I made that suggestion, because I want to make this suggestion: Plessie should have serious spell resistances or various weird item effects. Like, a Tier 4 party can probably all fly - what if Plessie can swim in a circle and one of the many bejewelled items stuck into its hide activates and a frickin' waterspout appears? Instead of giving it retaliatory intentional ranged attacks, hitting Plessie's hide with spells might run the risk of activating all these random things and making GOD KNOWS WHAT happen.

This is because Force Cage or Hold Monster might really, really heck up Plessie. Plessie should probably have multiple legendary saves too.

The bite attack should basically be a free grapple as the creature clamps down on someone and pins them in place. It should also do probably twice or three times as much damage - Plessie has great big teeths and only one bite a round after all, so it's got to be scary shit. I'd also say that it should miss for half damage if the players are on the boat, because a giant creature is slamming its head into the thing they're standing on even if it doesn't get them directly.  In fact some attacks that are miss for half damage might be a good way to evoke the size of a gargantuan monster.

Uh also +17 to hit is basically "hit on anything but a 1" against a number of melee classes. Monks CANNOT get an AC higher than twenty, for example. Think of the poor Rogues! Reducing the attack bonus but giving Plessie miss-for-half or save vs dex or save vs con type attacks might help make it a little harder for +3 plate magic shield paladins to just tank it all effortlessly.

Swallow attacks (if Plessie vanishes at half health, it leaves a surprised PC in the water), stunning bodyslams, or perhaps binding a creature up in its sinuous neck might also be fun flavorful attacks to add to Plessie?

Legendary Action: Breach should autocast Tsunami (8th level spell, PHB) for free. I know that you're using the Kraken stat block, FITE ME.

ALL boss monsters should have Bonus Actions and Reactions imo. Some thoughts: Bonus action might be a neck slap against a target other than the one Plessie bit at this turn. Reaction might be upon taking a ranged attack to do a defeaning scream that's an upcast Thunderwave at level 7 or 8 (8d8 or 9d8 in a 15 ft cube)

A critical hit or a hit past a certain damage threshold might knock a tooth loose from Plessie, to allow an overmatched party to still get the objective and flee.

THE KELPIES:

Are the Kelpies trying to kill the party members? If so they're in deep trouble, a couple of fireballs will take care of this problem right away.

Recommend you give them an attack or ability that can knock or entangle a character, and bonus actions to make an additional round of attacks. They should also have either a bonus or reaction, and maybe their wet nature gives them some resistance to fire damage? Do they know Counterspell? They might ought to know Counterspell. You might make it that the Kelpies AS A GROUP have four or five charges of counterspell they can use like a team-wide legendary reaction at their discretion when the party goes to cast magic.

The Kelpies need more + to hit, as well. They'll be hard pressed to hit anyone with AC 20+, unlike Plessie who will be hard pressed to MISS anyone with AC<20. Recommend something like +8 for the Kelpies.

The drowning hypnosis might ALSO be attempted on targets the Kelpies hit with their attacks, so that effectively it checks both AC and WIS saves. They're getting up in their victims' personal space and it's hard to look away and its a thematic ability.

If the Kelpies are not murderous but just territorial, they should try to flee (esp using their disguise powers) or bargain for their lives if they take significant casualties or they're all below half health.

MUSHROOMS:

These creatures all have intelligence and some of them have a peaceable nature. Can't we just talk about this with them? Can high-end druids take control or befriend them?

I think the other commentator's comments are on point. I also think that at Tier 4, ALL monsters should have AT LEAST one of bonus actions or reactions, outside of the situational Rot Touch.

I might point out that at this point all parties are going to have access to extremely powerful ranged attacks. What if the mushrooms can make an obscuring fog of spores to hide in if that happens? As faerie creatures do they have any illusory magic that can be deployed in combat to protect them from rangers with eleventy billion attacks or wizards with a one-fireball-fits-all solution to problems?

Attack bonus should be at least +8, giving a 50/50 chance of hitting AC 18 and a 25% chance of hitting AC 23.