Is this set of powers balanced?

Question

The setting of my campaign begins with the players waking up and escaping after being experimented on. The experiments give them a tattoo that they will progressively discover, grants them exceptional abilities at a cost.

My general idea is the tattoos power increase with level and enables them to do greater feats with lower costs as they "train". It will also be a driver of the campaign (they'll receive visions and, at the beginning, the tattoos will threaten their lives).

I'm trying to come up with a system that will not be too unbalanced (note the penalty is always applied in full at the beginning of the third round):

• Levels 1 to 3. They don't have control over their abilities. I (the DM) control exactly when they use it and how. Usually to provide epic saves on predefined situations to build up the story. Penalty: unconcious for 5 minutes, 15% lost hit points.
• Level 3 to 7. They can invoke it once every rest and 24h period. It grants them triple proficiency bonus on all rolls in the turn, accumulable to any other bonus and single proficiency and advantage on the following turn. Penalty: Stunned for 3 turns, 15% lost hit points.
• Level 7 to 15. They can invoke it once every rest and 24h period. It grants them triple proficiency bonus on all rolls in the turn accumulable to any other bonus and an extra action on that turn with no penalties. They gain single proficiency and advantage the following turn. Penalty: Stunned for 2 turns, 20% lost hit points.
• Level 16 to 20: They can invoke it once every rest and 24h period. It grants them triple proficiency bonus on all rolls in the turn accumulable to any other bonus and an extra action on that turn with no penalties. They gain double proficiency and advantage on the following turn. Penalty: Stunned for 1 turn, 30% lost hit points.

The idea is: they can use it to power up and perform incredible feats but it will come with a cost and let them unprotected and weak afterwards so they have to think twice about using it and when. The power can be invoked at any time, even in the middle of a foe turn in order to provide, for example, a bonus for a saving throw.

This is my first game on 5e and I'm using the tattoo as the main story driver but I'm afraid the "powers" will unbalance the game. Two questions:

• Since all players receive this: Is this balanced across the different classes / races or will say a Wizard benefit a lot more than a warrior?
• Is this balanced in term of story line or will it make the players too powerful and hence they'll tend to dominate any fight I'll put them in? What worries me here is I can't prepare every encounter assuming they'll use it so, ideally, the presence of these powers may turn a hard encounter into a medium difficulty one, but it would be bad if a "deadly" encounter becomes an easy one after using the abilities, then it will likely be unbalanced.

Answer 1

These tattoos are super overpowered, especially for any class with AOE.

Here's the basic formula you're using, I've simplified it, and am going to break it down to fit more with the 5e design patterns.

[...] They can invoke it once every rest and 24h period. It grants them triple proficiency bonus on all rolls in the turn, accumulable to any other bonus. [1-2x] proficiency and advantage on the following turn. Penalty: Stunned for [y] turns, [z]% lost hit points.

Sentence 1:

They can invoke it once every rest and 24h period.

Design:

This should instead be placed at the end of the feature and state: "Once you use this feature, you must finish a short or long rest before you can use it again." See the fighter's Second Wind feature. Additionally, invoking it should be associated with either a type of action (reaction seems to work best here) or a command word.

Balance:

There is nothing inherently imbalanced about this sentence.

Sentence 2:

It grants them triple proficiency bonus on all rolls in the turn, accumulable to any other bonus.

Design:

This definitely clashes with the design doctrine of D&D. Specifically stated in the basic rules:

Occasionally, your proficiency bonus might be multiplied or divided (doubled or halved, for example) before you apply it. For example, the rogue’s Expertise feature doubles the proficiency bonus for certain ability checks. If a circumstance suggests that your proficiency bonus applies more than once to the same roll, you still add it only once and multiply or divide it only once.

The reason this is the case is that D&D 5e has bounded accuracy. This term applies to skill checks, saving throws, and any roll of the d20 as well as attack rolls. Notice how 5e doesn't get you to ridiculous to-hit bonuses, skill check bonuses, and otherwise, like 3.5 or pathfinder do.

Because of this issue, I have nothing to base any design tips on, other than perhaps a redesign is in order.

Balance:

This is super overpowered as stands. "All rolls" includes saving throws, attack rolls, skill checks, and, most importantly, damage. The Great Weapon Master feat (PHB 167) trades 10 damage for -5 on the attack roll, and that's just for a single target attack. This feature grants anywhere from +6 to +18 to both. This is even more problematic for a spellcaster. For a sorcerer at level 5, there's much difference between casting a fireball for 8d6 and one for 8d6 + 9 per enemy. Compare this to the Evocation Wizard's Empowered Evocation feature, which just adds the Wizard's INT modifier.

Sentence 3:

[1-2x] proficiency and advantage on the following turn.

Balance and Design:

Same as 2.

Sentence 4:

Penalty: Stunned for [y] turns, [z]% lost hit points.

Design:

Here is a good place to talk about turns. "Turns" holds a very specific meaning for D&D. In combat, a round is 6 seconds, wherein each combatant takes 1 turn, all within 6 seconds. Every other feature or spell within 5e is very particular about when on the turns something ends. Monk's Stunning Strike feature, for example, stuns the creature until the end of your next turn. The shield spell, on the other hand, expires at the start of your turn.

A strict reading of the feature you've designed with the following initiative order goes as follows:

Wizard's (lvl 5) turn | the wizard uses this feature and casts fireball

Fighter's turn | the wizard is now on the second sentence

Baddie 1's turn | the wizard takes 15% damage and is stunned

Druid's turn | the wizard is still stunned

Baddie 2's turn | the wizard is stunned

Baddie 3's turn | the wizard is no longer stunned

If you're determined to keep this feature, I'd recommend something like the end of haste:

When the spell ends, the target can't move or take actions until after its next turn, as a wave of lethargy sweeps over it.

Balance:

The balance of this feature heavily depends on how many baddies you put in the initiative order. If all of the goblins go at the same time, the feature is heavily punishing. If gobbo 1 goes, then gobbo 2 goes, etc, then it's not a punishment at all.

Additionally, the %hp loss punishes high hp people much more than low hp people. Don't worry about equaling out the HP loss for each character. There's a reason that enemy's attacks don't do %hp damage in D&D.

Final thoughts:

You're new to D&D, and it's great that you're being super creative with this kind of thing, but I'd suggest something much more simple (mechanically) for your first run. Maybe something like: The tattoos allow you to cast a spell once per short or long rest, depending on your level. Alternatively, you could look at the Artifacts and Sentient Magic Items sections in the Dungeon Master's Guide.