How powerful is this homebrew Barbarian Path of the Colossus compared to other martial subclasses?

Question

As DnD 5e has aged it's become more clear that Martial classes lack a fair amount compared to their spellcasting companions. They offer less utility, fewer defensive options, and in some cases fail to perform offensively. So I started looking at ways to start pushing back towards a more even scale. The easiest of which ended up being the barbarian. This class thematically has always been combat forward with little utility. So what better way to start looking to make a strong subclass than to expand on a simple theme. Which brings us to now.

How strong is the Barbarian Path of the Colossus compared to the strongest Martial subclasses?

Simply put, I'm not looking to make something balanced to the other Barbarian subclasses, most of which rank somewhere between C and D tier in my opinion. I'm looking to check the balance of this subclass against high tier martial subclasses like the Echo Knight or the Rune Knight. This way, my table can begin to create a more level playing field. With that in mind, the subclass:

Path of the Colossus

Barbarians who choose to follow the Path of the Colossus strain their bodies to untapped physical heights. They strive to become a juggernaut on the battlefield and a bulwark outside it. Seeking at once to temper the Rage and to heighten it. Those who follow this path are among the most spartan of their tribes, often abstaining from lavish celebrations instead choosing to spend time investing in the everlasting pursuit toward physical perfection.

Spartan Rage

Starting at 3rd level, when you enter a rage you can choose to temper or heighten your rage. You maintain this effect until the rage ends and can choose each time you enter into a rage.

Heighten. You double the bonus damage from your Rage Damage. However, when you exit your rage you gain a level of exhaustion.

Temper. You halve the bonus damage from your Rage Damage. However, you gain +1 AC and temporary hit points equal to your Barbarian level x 5. The AC bonus and temporary hit points last until your rage ends.

At 10th level, if you heighten your rage you no longer gain a level of exhaustion and if you temper your rage you no longer halve the bonus damage from your Rage Damage.

Heightened Physique

Starting at 3rd level, you become much more skilled in feats of strength and agility. You gain proficiency in Athletics and Acrobatics. If you already have proficiency in these skills, then your proficiency bonus is doubled for any ability check you make that uses either of these skills.

Titan's Grip

Starting at 6th level, you ignore the two-handed property of weapons. Additionally, you have advantage when making attempts to grapple or resist being grappled.

Physical Mastery

Starting at 10th level, you gain a climbing and swimming speed equal to your movement speed. Additionally, you no longer take a point of exhaustion when you heighten your rage nor halve the damage from your Rage Damage when you temper your rage.

Colossal Strikes

Starting at 14th level, while raging, when you deal damage with a melee attack that uses strength you can roll an additional weapon dice and you can make another attack with the same weapon against a different creature that is within 5 feet of you.

Answer 1

This is too strong for an already strong combat class

This large surveys shows several barbarian Paths as a Tier A or B+ option, so I think your premise that the Bararian subclasses are all Tier C or worse and Barbarian needs a new Path to compete for playablity is overstated.

There may have been a bit of power creep with new subclasses since the survey was done, but it does not seem there is a real problem with Barbarians that needs to be fixed, and by making something much stronger, you run into the risk to be unbalanced. In fact, its probably impossible to balance all the classes perfectly, and if you continue to invent new, stronger subclasses to always match the very strongest single one you can find, you are on a one-way road to power creep that will entirely invalidate the core rule subclasses.

If your focus is on combat as the role for he Barbarian, I'm not sure how you arrive at the conclusion that it is a tier C (ie. weaker than average) class. Also, this class tier list shows barbarians as tier A, the "abosolute best" in the Damage Per Round and Tank combat caterories. So this can hardly be based just on combat and damage output or resiliencs.

To demonstrate this: a barbarian with Great Weapon Master and Polearm Master, no matter what path, is already one of the builds that can deal among the highest amount of reliable damage for a melee combatant. They can use Reckless Attack to effectively offset the negative to-hit from the -5/+10 from GWM. A variant human with those feats by level 8 can have an expected damage output of:

65% (typical to hit chance) x ((2d6+4(Strength)+2(Rage)+10(GWM)+ 2d6+4(Strength)+2(Rage)+10(GWM)(extra attack) + 1d4+4(Strength)+2(Rage)+10(GWM)(bonus attack) ) + 1/20 x (2d6+2d6+1d4) (criticals), or about 43 points of damage dealt.

If you compare this to the average damage per level various classes deal, you can see this is very high. The referenced builds include not only optimized ones, so their 20 points of damage delivered for the top quartile of builds may be low as a reference point to compare to highly optimized ones. But even a group of 7 builds specifically made to optimize for damage only averages about 30 points at 8th level (they did not include GWM, which accounts for the gap).

So barabarians can be very effective as damage dealers. However, you belabor that they are one-dimensional. And your design does next to nothing to diversify what the barbarian can do, it is nearly entirely combat focused on adding more damage, adding more attacks, adding more hit points, adding stronger grappling. Even where you are not single mindedly focusing on combat application, granting proficiency or more likely, the equivalent of Expertise, to Atlethics and Acrobatics, you end up essentially fueling better combat maneuvers and grapple attacks.

A class that is already among the top damage dealers really has NO NEED to add even more gratuious damage and attacks. Your Path by level 14 would add an additional damage dice, an additional attack, an additional 3 damage to each attack. Assuming there are enough opponents, with a greataxe this would add 2d12+5(Stength)+6(Rage)+10 from the added attack, and 1d12+3 directly to each of your 3 attacks, or another 40 damage, basically doubling that already high output.

At the same time, there is little to compete with the versatilty that martial classes with limited spellcasting ability or other cool special features like the Rune Knight have. Effectively you have a better climb and swim speed to help somewhat with exploration, and expertise on Athletics and Acrobatics. Compare this with other classes, who at least get some tools for the exploration and social interaction pillars of the game. For exploaration, this is not much better than for example the Path of the Beast, which with Bestial Soul gets similar movement benefits.

Answer 2

TL;DR: Tone down the durability buff of Tempered Spartan Rage and the damage output of Colossal Strikes, and you'll be balanced against other top-tier martial subclasses

Since Rune Knight and Echo Knight were the examples you offered, let's compare your subclass to those two, as well as the other three martials that I would consider to be the strongest: Gloom Stalker Ranger, Conquest Paladin, and Hexblade Warlock.

Combat Damage

To evaluate the highest combat damage achievable, I will assume each subclass takes the Polearm Master feat at level 1 as a variant human, the Great Weapon Master feat when they hit level 4 with their main class, and gets advantage from a relevant source:

• Our fighters and paladins get advantage starting at level 7 by multiclassing as a Barbarian for 2 levels starting at level 6.
• Our ranger gets advantage consistently starting at level 3 via a combination of the entangle spell and Umbral Sight.
• Our warlock gets advantage starting at level 3 via the darkness spell and the Devil's Sight Invocation.
• Your barbarian gets advantage starting at level 2 with Reckless Attack.

With that in mind, here are the damage per round tables for these builds (assuming a 60% base hit chance less any missed or delayed ASIs for feats and multiclassing):

...or a graph to show the data side by side:

As a brief summary, your subclass is middle of the pack until it spikes dramatically at level 14 to lead the pack for the remainder of the levels.

Effective HP

To evaluate effective HP, I assumed each subclass took defensive options where possible (such as the ranger taking the Defense fighting style since they didn't have an offensive option) and maximized Constitution after their primary ability score.

The formula I used was created by reddit user TheRainyDaze in this post, and makes a few assumptions, but I find them reasonable:

• 75% of damage is physical, divided evenly between bludgeoning, slashing and piercing

• 25% of damage is elemental, divided evenly between all ten elements

• Characters are being attacked by creatures with attack bonuses and spell DCs that follow the DMG guidelines for a creature with a CR equal to character level – 2

• 80% of attacks are made against AC

• 10% of attacks require a Dexterity Save

• 5% of attacks require a Wisdom Save

• 5% of attacks require a Constitution Save

• A negligible amount of attacks require a Charisma, Strength or Intelligence Save

Using these metrics, we get the following EHP tables (I treat imposing disadvantage such as with darkness or Umbral Sight as comparable to +5 AC; although this does overestimate the Gloomstalker ranger as you won't always get Umbral Sight):

...or the following graph:

Once again, your barbarian follows largely the middle of the pack until between level 16 and 17. In this case, this is because more frequent rages for resistance continues to allow the subclass to scale in EHP while the other subclasses don't have any scaling defensive features.

Conclusion

Looking at the graphs, your barbarian is able to keep up with the best defensive options that use invisibility (from Umbral Sight or darkness) to avoid taking damage. Your subclass accomplishes this by having more HP, more frequent rages for resistance, and by wielding a shield thanks to Titan's Grip to keep AC high.

Your barbarian is also able to keep up with the best offensive options of the two fighters who get the improved Extra Attack at level 11 to compile the bonuses from Great Weapon Master. Your subclass accomplishes this by effectively gaining an extra attack and then some at level 14.

Barbarians are supposed to be especially durable, but should not be able to keep up with the damage of optimized fighters (with their lesser durability) in terms of damage. As such, the level 14 feature is clearly too strong in terms of damage output.

Durability

Once you've addressed the damage output, let's make sure the durability is in line. Since we aren't focusing on damage, let's evaluate effective HP using the Tempered rage. Here is the new graph:

As you can see, the durability of the subclass is also too high. I would tone down the Tempered rage as well, so that it doesn't outpace other options in terms of durability.

Recommendations

Here are the changes I would make:

Tempered Spartan Rage: remove the temporary hit points; only granting the extra AC. This is already enough to be more durable than all the other build we've looked at, but at least is in a more sensible range since it sacrifices damage to accomplish it:

Colossal Strikes: remove the extra attack and add a non-damage effect such as a saving throw to avoid having reduced speed. This results in the following DPR graph:

With this, you can reign in the absurd durability and damage output these two features create.