I have an idea for a campaign for my Party [They are fine with Homebrew. So, that won't be a problem]. I am looking to make this boss memorable compared to my other bosses.

I have an idea on having my final boss being completely invulnerability to all damage types and sources. The only thing that can damage it, is it's own attacks. As the creature is completely indestructible due to a power energy shield around it. I should clarify that after this shield goes down, the boss gets stunned or something and the Party can attack the boss with any attack. As this boss will not have any immunities or resistances to accommodate for it's "invulnerability" mechanic.

I have two main questions. The first is if this is even a good idea, or if I should just stick with the 'classic' stab the BBEG until he dies. The second question is how should I get this to work with the boss's current abilities. To help you get a better understanding, I will provide her abilities below and my thought process behind it (If something doesn't look good, you can let me know about that too).

A General Description of the Boss's abilities and attacks

Umbrakinesis

1. Shadow Manipulation: The boss can manipulate shadows to create barriers, shields, or even solid objects. This could provide her with a form of defense or be used to obstruct the vision of players.
2. Darkness Infusion: The boss can infuse her attacks or spells with pure darkness, causing additional 'shadow' [necrotic] damage.
3. Shadow Teleportation: The boss can teleport between shadows, allowing her to move swiftly across the battlefield or evade attacks. This could make her a highly mobile and elusive opponent.
4. Eclipse Burst: The can create localized eclipses, plunging the battlefield into temporary darkness. During this time, she gains enhanced abilities, making her more formidable but also more vulnerable once the darkness lifts.

Dark Wave Generation

1. Fear Effect: The boss emits a passive field or aura of fear around herself. Creatures caught in this wave could experience varying degrees of fear, affecting their actions and potentially forcing them to move away from the boss.
2. Suffocation Mechanism: The dark energy coming off the boss could create an environment where the air becomes thin or tainted. Creatures within the wave might need to make Constitution Saving Throws or risk suffocation until they back up.
3. Situational Awareness: Players who recognize the signs of Dark Wave Generation (perhaps a subtle darkening of the surroundings or a distinctive sound) could have the chance to prepare or counteract the upcoming wave.
4. Countermeasures: Consider allowing players with appropriate spells, items, or abilities to protect themselves or their allies from the fear and suffocation effects [like the Air Bubble spell]. This adds a layer of strategy to the encounter.

Illusions Generation (Can only make illusion copies of themself)

1. Player Interaction: The illusions react differently to certain attacks or abilities. Clever players might figure out ways to distinguish the real boss based on these interactions.
2. Temporary Vulnerability: The illusions could have significantly reduced health and disappear upon being hit.
3. Auditory Clues: There may be auditory clues that reveal the real boss. The boss might make a distinct sound or have a unique vocalization that players can listen for amid the chaos of illusions.
4. Limits: The boss will not be able to constantly create illusions of themselves. There is a maximum and a 'recharge' to the ability.

Energy Balls

1. Variety of Effects:
   1. Damage: Energy balls cause different types of damage upon impact.
   2. Status Conditions: Others may inflict status conditions, such as blindness, slowed movement, or vulnerability to certain types of attacks.
   3. Debuffs: Energy balls might apply debuffs like reduced accuracy, lower damage output, or increased vulnerability.
2. Distinguishing Characteristics:
   1. Each type of energy ball would have a distinguishing characteristic (like its color), so players can quickly identify the potential threat they pose.
3. Cover Mechanics:
   1. The arena could have some ways where the Party can use cover. Players can use this strategically to shield themselves from the energy balls. (This can encourage movement and positioning during the fight.)
4. Spell Interactions:
   1. Certain spells or abilities may interact with the energy balls. For example, certain protective spell might absorb the effects of an energy ball, turning it into a temporary buff for the player.
5. Predictable Patterns:
   1. Maybe there will be a pattern in the boss's energy ball attacks. This could help players anticipate when and where the next set of energy balls will appear. {Ex. Green, Red, Blue, Purple, Green, Red, Blue, Purple, Green, etc.}
6. Scaling Complexity:
   1. New energy ball attacks may appear as the battle progresses or when the boss reaches certain health thresholds.

Shadow Attack

1. Blinding Light:
   1. When the boss triggers the Shadow Attack, a blinding light fills the arena, creating the shadows of the players. The shadow's of the Player's will come to life and 'detach' from the Player.
2. Shadow Connection:
   1. Damage to a player's shadow is reflected on the actual player. The shadows essentially become a vulnerability that players need to protect.
3. Dodge Mechanics:
   1. Players must actively dodge with their own shadows to avoid taking damage. This adds a skill-based element to the encounter, promoting movement and agility. As however or wherever the Player moves, the Shadow will follow. (Ex. If the Player jumps, the shadow will jump at the same time.)
4. Visual Clues:
   1. There will be visual cues or patterns to signal where a player's shadow will appear. This could be in the form of a brief shimmer on the ground or a subtle change in lighting.
5. Environmental Elements:
   1. The area may have elements that players can use strategically to break line of sight with the boss. This might include pillars, walls, or magical barriers. This is to they are not casting a shadow from the flash of light.
6. Temporary Shadows:
   1. Shadows will persist for a short duration before fading away.
7. Teamwork Dynamics:
   1. This can hopefully encourage players to communicate and coordinate movements to help each other avoid taking damage from their shadow.
8. Scaling Difficulty:
   1. More shadows may appear as the battle progresses or when the Boss reaches specific health thresholds.
9. Player Shadow Abilities:
   1. Players may gain temporary buffs or unique abilities when interacting with their own shadows. This could add a layer of strategic depth to the mechanic.
10. Feedback Mechanism:
    1. There will be a clear 'feedback' sign, such as visual and auditory cues, to let players know the boss is about to activate this ability.

Charged Blasts

1. Charging Animation:
   1. The boss would have a distinctive and visually noticeable animation when preparing for the Charge Blast. This provides players with a clear signal to react.
2. Casting Time:
   1. There will be a 'casting time' for the Charge Blast, giving players a brief window to respond. This could be a few seconds during which players need to make a strategic decision.
3. Area of Effect:
   1. This will be a cone area of effect attack.
4. High Damage:
   1. The Charge Blast is a high-damage attack to emphasize the threat. Players who fail to dodge or mitigate the blast would suffer significant consequences.
5. Interrupt Mechanism:
   1. There will be opportunities for players to interrupt or disrupt the charging process. This might involve interacting with the environment, using specific abilities, or coordinated team actions.
6. Risk-Reward Scenario:
   1. There may be a risk-reward dynamic. For example, interrupting the Charge Blast might expose the interrupting player to some risk, but successfully doing so could provide a temporary advantage.
7. Cooldown Period:
   1. After executing the Charge Blast, the will enter a brief cooldown period where she's vulnerable. This creates strategic moments for the players to capitalize on.

I'll be honest, I didn't read all the abilities thoroughly. There's more there than in most stat blocks I've seen, which is a little worrisome. 

What level is the party? How many of them are there? How much and what kind of damage will the boss deal to players each round? How much HP does she have? How difficult will it be for the players to destroy her invulnerability shielding? A list of vague abilities doesn't really paint a picture of how feasible this fight would be.

The concept of having the boss be invulnerable to damage for a little while is well within D&D's normal mechanics if you just cast Invulnerability on the baddie, or homebrew the Globe of Invulnerability spell to include all damage. I've done something similar, and had four braziers powering my BBEG's shield. My players had to destroy them before they could really lay into her, and I started weakening her defenses as they systematically took them down. 4 braziers was full immunity, 3 let spells through at half damage, 2 let physical attacks through at half damage, and 1 only provided a buff to her AC. 

Puzzle encounters like this and what you're describing can be fun. Think through more of the damage and player experience as you prep this. 

I listed only 6 of the bosses main abilities. The numbered list is just me explaining the idea on how it works. If you don't want to read it, I understand. I wrote it just in case it could help.

The Party's level would be at 10 - 11. Moreover, there are only two Party members within the Party, as the others quit the campaign because other things came up that needed to deal with. However, to make up for it, we all agreed to add an DMPC to help them out if needed. (This 'Helper NPC' will be a Bard).

The Boss will mainly be dealing necrotic, force, and magical slashing damage. As her attacks are based off Dark Energy and Dark Matter. The amount of damage she deals depends on the attack she uses (I haven't really decided on how much damage each attack will deal yet.) When it comes to destroying her invulnerability shielding, that was the main question. However, so far I was thinking that the Party will need to hit the Boss with her own Energy Balls to disable it. When it comes to how many, I have no idea yet.

Should I iron out and finalize her abilities first, then send them here before I can make a choice? As I was thinking that I can model the bosses attacks to work with the invulnerability. (Make it where the Players can disable it by redirecting or using her own attack against her.)

As a note, based on just the abilities and a flat simple line, the Boss is way and far above the CR range for 10 to 11 level PCs.

THey are more akin to a 17 to 20 level character. at least imo.

Which is fine if that's what everyone at your game wants and enjoys.

I will note that most of those abilities do not actually give the players a chance to cause harm if played as a straight person -- because no one is going to put themselves at risk in combat. However, as a video game boss, the mechanisms are fine, but actual play will be a bit rough for the players.

I will note that I wouldn't use a being like this in my games, even at 20th level. They are, for all practical intents and purposes, in the way I run my bad guys, literally impossible to defeat under any circumstances short of a wish -- for the players to win, the villain has to suck at being a villain.

When I want to do an epic fight, I typically break it into stages, with each stage having a particular goal. Using that framework, you could have a first stage where the goal is not to harm the BBEG, but to disable whatever is protecting them. Then stage 2 is the actual fight.

I think this might go over better than "redirecting her attacks is the only way to hurt her," primarily because as a player I like to put my own skills and features to use. If I've been honing my bladework for years in order to defeat the six-fingered man who killed my father, I'd be a little disappointed if I couldn't actually use that bladework in the final fight.

Hitting a boss with its own energy is not really a D&D mechanic, so unless that's a homebrew rule you already use at your table, or you very clearly warn the players (through in-game research, rumors, showing it with NPCs who try this within view, etc.) that this is the way forward, they are probably not going to figure it out - meaning AEDorsay is right and the boss is effectively impossible to beat. 

Even with the 6 abilities you shared, the boss is unreasonably strong with a ton of battlefield control and maneuverability that the players can't really counter. Depending on how the fear and darkness effects work, the players may not even be able to attempt normal attacks, let alone energy ball redirections. Normally, aura effects have a save attached, or a limited range of effect. Without Devil's Sight or Truesight, they will be blinded, which means all of the boss' attacks will be at advantage and all their attacks will be at disadvantage. Fear effects will cripple any melee fighters, and if they are also under the Frightened condition, they'll have disadvantage on any attacks whenever they do see her. 

I think you have a cool concept that, with some adjustments, could turn into a very memorable fight. First, though, I recommend thinking through the actual mechanics of the abilities (damage per attack, save DCs, number of attacks per round, legendary actions, hit points, effect ranges and durations, etc.) and running a test combat by yourself to see how the PCs would fare. Remember to put yourself in their shoes when you test it, because they aren't going to know the secrets and tricks to beating her. After a test combat you'll have a better idea of how to tweak the boss to be a fight that's hard but also winnable. Looking at existing stat blocks (say for Acererak or an ancient dragon) will also help determining a more balanced, challenging encounter. 

I have a better idea on how the bosses abilities work now. They will be listed below. With these attacks, would this boss sound doable? I'm mainly referring to the 'Invulnerability' trait. (The Party's level would be 10-12 and they have a few magic items to help them.) If you think you have a better idea on some of the boss's abilities, I would be glad to hear your thoughts. [One thing to note, is that the boss's Supernova (Charged Blast) and Shadow Attack is intended to be extremely powerful. However, it takes a while to charge and is obvious that something big is coming. So, the Party has a choice. Full on attack while the boss is charging to attempt to break it, or run for cover and wait for the attack to pass.]

• Umbrakinesis Barrier: As a bonus action, the boss can create a barrier made of shadows with 150 hit points. The barrier covers a 30-foot radius and lasts until the boss's next turn.
• Dark Wave Generation: Once per round, at the cost of a legendary action, the boss can unleash a wave of dark energy in a 60-foot radius. Creatures within the area must make a DC 18 Constitution Saving Throw or be Frightened for 1 minute. Additionally, creatures who fail the Save begin suffocating and must make a DC 18 Constitution Saving Throw at the start of each of their turns to end the effect. Constructs, undead, and creatures that don't need to breathe are immune to this side-effect.
• Invulnerability: The boss is immune to all damage types except radiant and force. However, every time she takes radiant or force damage, its invulnerability is suppressed until the start of its next turn. Breaking the boss's invulnerability while she's charging an attack will cause the boss to lose it's current charge and become stunned until the start of its next turn.
• Illusion Rush: Once per round, at the cost of a legendary action, the boss can create illusory duplicates of herself. The duplicates have only half of the boss's hit points and deal half damage. They disappear after taking any damage.
• Invisibility: As a reaction, the boss can turn invisible until the start of her next turn by bending light around herself.
• Energy Balls: The boss can launch up to three energy balls as ranged spell attacks. Each hit deals (4d8) Necrotic damage, and the target must make a DC 18 Constitution Saving Throw or suffer a random negative effect (e.g., blindness, deafness, paralysis) for 1 minute. The creature can repeat the save at the start of its turn, ending all current negative effects at once on a success.
• Supernova (Charged Blast): The boss charges for 2 rounds, then releases a spiraling energy wave in a 120-foot cone that lasts until the start until the bosses turn. Creatures in the area must make a DC 28 Dexterity Saving Throw, taking (10d10) Force damage on a failed save and being launched back a distance equal to half of the creature's current distance from the end of the blast range. [Example on how it works: If a creature is located in front of the boss when this goes off, they are 120 feet away from the end of the blast range. Meaning they will be launched 60 feet away in the direction of the blast. If a creature is located 90 feet away from the boss when this goes off, they are 30 feet away from the end of the blast range. Meaning they will be launched 15 feet away in the direction of the blast.] Getting a successful save deals half the damage, and only get pushed back half of the distance.
  • The fall damage from being launched will still apply if needed. Being pushed doesn't give fall damage, as the creature is only being pushed back.
• Shadow Attack: Once per round, the boss takes a moment to charge up a large AOE flash. On its turn, the boss begins to emit a blinding light, signaling the impending attack. The charging process lasts for 1 round, during which Players have a chance to react. On the boss's next turn, it releases a flash of blinding light in a 90-foot radius. All creatures within the caught within the light of sight of the boss/flash must make a DC 25 Dexterity Saving Throw.
  • Success: The creature avoids the blinding flashes effect, and its shadow remains inert.
  • Failure: The creature is blinded until the end of its next turn, and its shadow comes to life. This shadow replicates the creature's movements and actions but it not initially hostile.

Once a creature's shadow becomes animates, it mimics the creature's actions but does not deal damage or can cast magic. However, if the boss successfully hits the animates shadow with an attack, the corresponding Player takes damage.

Shadow Interaction: The animated shadow appears in a random location with 30 feet of its 'owner'. The shadow moves with its associated creature, copying its movements exactly. If the Player jumps, dodges, moves, etc., the shadow replicates these actions. If the animates shadow is hit by an attack, the corresponding Player takes the damage. The shadow has hit points equal to half of the Player's maximum hit points and is destroyed if reduced to 0 hit points. Another thing to note is that this animated shadow cannot travel through objects. However, if the shadow is running against something, the original creature is not hindered in any way; the shadow is just stuck wherever it is. Meaning Player can strategically use this to get their shadow close to them. If the owner of the shadow grabs their own shadow, it harmlessly disappears.

My 2cp worth...

Sounds like you just need to use the lvl 9 Invulnerability spell to give the BBEG plenty of time to monologue and for the players to question what the hell is going on, toss in some uses of Arms of Hadar, Eldritch Blast with Repelling Blast, Magic Missle, Melf Minute Meteors and Wall of Force, maybe some uses of the Mislead spell or Misty Step for teleporting shennanigans and toss in some use of the Trickery Domains "Invoke Duplicity" power and you have pretty much all the powers you mentioned.

You could even tack on the Abjuration Wizards "Arcane Ward" power to bolster itself if the Invulnerability wears off.

im with AEdorsey. consider dropping some abilities, and the only thing i see that will drop their energy sheild is the dragonball attack and how would they redirect that exactly?