Ritual of Binding

The following information was provided by your friendly local lich at The Amber Temple. This ritual should be performed after the Ritual of Summoning, unless you've just performed that one in a town you really dislike.

Entrapping the Dark Power

This ritual requires a chunk of pure amber at least one inch in each dimension, as well as at least three mages maintaining continuous, uninterrupted casting of up to one minute. When the Dark Power Power arrives, it will not behave peacefully. Though the nature of the Dark Powers cannot be defined, safe to say it is akin to battling a manifestation of divinity or purest evil. We believe that the manifestation of the Dark Power summoned by the previous ritual does not summon it in its entirety, but an aspect of the Power, akin to an avatar of a god. However, in the words of one mage who has participated in several of these ceremonies and lost body parts accordingly, “It’s like dealing with a tarrasque.”
As with the previous ritual, one caster must take the lead, deciding what level of magic to expend. Depending on the success of the previous ritual, the task ahead of you may be more or less difficult. There are reports of the Entrapment Ritual taking less time following particularly successful performances of the Summoning Ritual.
As before, at least three mages must maintain Concentration on the ritual for the duration. The lead caster must not lose Concentration at any point, or the ritual will fail, and must begin anew.
The Dark Powers are intelligent, and will know what targets to focus on. They have been bound by these means before. It is not enough to have guardians present to attack the Dark Power when it manifests, or to try to distract it. You must find means to help safeguard the casters, as the task of maintaining Concentration when struck by this manifestation is one to challenge even the most accomplished of mages. Protection spells, as well as anything that will magically compel the creature to attack other targets, or to have difficulty striking a specific target, are key.
All casters participating in the ritual must be able to see the Dark Power for the duration of the casting. They must remain on the same plane of existence as the Dark Power, and must not be behind any barriers, such as Walls of Force, which would prevent their magic from reaching through to bind the Dark Power.
Aside from maintaining Concentration on the ritual for the duration, the lead caster must channel their abilities into sundering the creature, extracting part of its essence to entrap it within the amber. If the lead caster is unable to succeed within the minimum time required for the ritual, they can continue to make their attempts, but even if the caster successfully channels their energy thrice, the ritual cannot end until the minimum casting time has passed.
At the end of the ritual, if it is successful, the amber chip will expand into an amber sarcophagus, with a shard of the Dark Power trapped within. However, the wounded manifestation of the Power will remain, and must be dealt with. In its weakened state, it will be easier to strike blows upon, and less hardy, but it is still a formidable foe. Once it reaches a certain threshold of damage — it is unknown even to us if it is weakened to the point of temporary unconsciousness, or simply reaches a level of pain it finds unacceptable — it will dissipate, withdrawing to the aether where all the Dark Powers reside, in its weakened state.

In Mechanical Terms…

At a base level, this ritual requires one minute of continuous Concentration by the lead caster, and at least two other casters maintaining Concentration at any given time, to be a success.
The success of the previous ritual based off those special parameters can reduce the necessary time for this ritual to take by up to half.
This is while you will be in active combat with something that someone compared to a tarrasque. However, it’s reasonable to assume that was an exaggeration. That said, tarrasques have five attacks per turn for an average of 148 damage in a single turn, along with Legendary Actions. They have 676 hit points and an AC of 25. So while I’m not throwing something of equivalent strength at you because if I gave you a tarrasque you’d die, it’s reasonable to assume you’ll be dealing with something with a high AC, a fuckton of hit points, multiple attacks, and Legendary Actions.
Rest assured that whatever the stats for this Dark Power are, I won’t give you something that deals so much damage in a single blow that making the Concentration checks will be impossible, but it will be difficult, which is why you’re going to want to take the Concentration check modifier of your lead caster into consideration.
However, you have to balance that with the spellcasting ability of the lead caster, as the three checks they need to make — against a high DC — will be based on their spellcasting ability and their proficiency bonus, essentially spellcasting attacks.
Keep in mind that how you handle the previous ritual can potentially lower the DC the lead caster needs to hit with these checks.

• So, for example, Azuth has a much lower Concentration check than Ri-An or Ayduin, only a +2 — +4 if he gets his shit back
• But he has a higher spellcasting attack than either of you (+11 right now, +13 if he gets his shit back, as compared to the +8 you’re both soon going to hit or the +9 you may hit by the final battle)
• I’m giving Azuth as an example because he’s the most powerful friendly mage y’all know and he has pretty standard archmage stats; for the sake of you being able to strategize, when we close in on the endgame and you’re planning how you’ll handle these rituals, I will give you this basic information on any of your spell-casting allies you request, so you can decide who you want to use and how — I know you’ll be sharing those mechanical details of your own abilities with each other, so you might as well have them for your NPCs, too

Don’t forget that the lead caster will get a bonus to their Concentration checks if there are more people maintaining Concentration with them.
Also don’t forget the matter of hit points — even if your caster makes all their Concentration checks, if they pass out from the damage, the ritual fails.
If a lead caster has a high AC that will help, but again, you’re dealing with the avatar of something powerful enough to make people immortal and bend reality around them to create eternal demiplanes of torture, so I hate to break it to you, but even Ri-An’s AC isn’t liable to count for much in this fight.
If the lead caster innately has or has been given some sort of resistances, that will help, too.
Having as many healers or healing tools on hand as possible will also likely be to your benefit, especially anyone capable of big heals to counter big hits. However, anyone who’s healing isn’t helping with the ritual.
It is reasonable to assume that the Dark Power that has given Strahd his powers will have similar powers, so strategize accordingly in what tactics you would use to distract it/to beat it down after the ritual.
There will not be the expectation that you take on this fight directly after your fight with Strahd — the Dark Power not being summoned unless and until you go out of your way to summon it gives you some control over the timing — but the longer you wait, the more time any of his remaining allies will have to plan, so take that into account.
As before, any casters participating in the ritual cannot use actions, bonus actions, or reactions while concentrating on the ritual. However, if they’ve already lost Concentration — from damage or something else — they can use their reaction and their bonus action for that round, though they will have to use their action to resume Concentration on the ritual, or wait until the next round to do so.
Your biggest priority during the ritual should be doing whatever you can to prevent the casters from taking damage, and/or minimising any damage they take. In particular, the lead caster must be protected, because if the lead caster loses Concentration at any point, you have to start all over again. The others can lose it then pick it up again — provided there are always at least three Concentrating — but the lead caster is the lynchpin.
As before, the lead caster will get a bonus to their Concentration checks based off the number of casters actively maintaining Concentration with them.
Whenever there’s a total of four or more casters Concentrating on the ritual, all casters will get advantage on their checks.
In this circumstance, I will allow you to use your reactions to literally throw yourselves in front of the casters as needed and take damage in their place. However, you’re also going to want to find other ways of protecting them and/or distracting the Dark Power in question.
This is also the best possible time to make use of as many of your allies as possible. Even after the ritual is complete, you need people making strikes against the weakened Dark Power to convince it to dissipate. (Thankfully, once the ritual is complete, your casters will be free to join you.) So far as you understand the ritual, Strahd will not come back once the Dark Power has been sundered/contained, but you don’t really want to leave the avatar of the Dark Power running around Barovia or beyond wreaking havoc. If you don’t survive the first attempt at a battle, it’s not like you can’t take a rest and try again — but that will give the avatar time to regroup, too, and fuck shit up in the meantime, so the more people you have at your side ready to help both guard the mages and take it down, the better.
To cover the allies you have who you have confirmation would qualify to participate in either ritual, you have seen the following people cast 3rd-level spells or higher:

• Azuth (Concentration checks: +2/+4)
• Oileán (Concentration checks: +2)
• Ezmerelda (Concentration checks: +3; may have damage resistances)
• Victor Vallakovich (presumably; teleportation) (Concentration checks: +0)
• Godfrey (Concentration checks: +8; has damage resistances)
• Neferon (Concentration checks: +2; has damage resistances; may or may not be willing to leave the Temple)
• The Lich (will not leave the Temple, confirmed)

This list is not necessarily exhaustive, but they’re the allies you have confirmation of.
In summary, the phases and requirements of this process are:

• Summon the Dark Power, ideally in circumstances where you don’t expect any of Strahd’s remaining allies to attack you
• Entrap the Dark Power by:
  • The number of total casters maintaining Concentration on the ritual never dipping below 3 for the minimum duration (anywhere from 10 rounds to 5 rounds depending on how the first ritual goes)
  • The lead caster maintaining Concentration continuously for the minimum duration of the spell
  • The lead caster making three successful spellcasting attack rolls against the Dark Power during the ritual, against a DC that will be reduced depending on the success of the first ritual
• Fight the weakened avatar of the Dark Power until you reduce it to whatever threshold it dissipates at
• And, y’know, theoretically take the amber sarcophagus back to the Amber Temple so it can hang out with all the other trapped evil there