Yeah, doing the work on it, Monk/barbarian multiclass, monk swapping discipline of the dragon for the path of destruction, is utterly insane.

The core of this is Great Cleave, which is very powerful in its own right; doubling your attacks, essentially, which is a very powerful ability, and should probably be the capstone over the pitiful "enemies take 20 damage if they are near you" capstone.

So you pick up a spiked chain, make it your weapon for path of the serpent, and at level 20, you get:

A 5d8 base damage spiked chain with all the normal goodies. This is the least broken part.

Flurry. -1 to your attacks for an extra attack, doubled with great cleave. This means we are up to five attacks, ten if they all hit, and cleaves are taken at your highest BAB, so if you *do* hit, then you're probably going to hit the cleaves as well. Even more fun, if each great cleave is its own attack action (the term isn't defined) then you can flurry each of them for two attacks per attack you hit, luckily nonrecursively.

Hydra's Heads let you make mirror images pretty much guaranteed every round, at the cost of your swift action. You can also get another hit (and then another from cleave) from curse of stolen time.

You can attack all enemies in your way on a charge, possibly? (again, language isn't clear on attack action) attacking each one twice via flurry, using Shiva's charge, then great cleave from all those attacks to taste. Your charge has +70 foot movement, so you're pretty well set to charge trip. Normally, I wouldn't be sure how the great cleaves work, but since you can attack any time during your move with deftly striking, I'm pretty sure you can use your cleaves on any of the targets you choose, although that makes little sense, but even being forced to just cleave those who are in range at the time is incredibly powerful.

Then, you can use Thousand Sudden Cuts to double, then quadruple (via cleave) your attacks for the round.

Possible situations that are realistic:

You go all out and full attack somebody. You attack five times, cleave five times, then attack five more times and cleave five times. 20 attacks, or 30 if you can flurry your cleaves (which I say, for obvious reasons, you shouldn't be able to, but attack action is not defined at all).

You charge towards the big bad, with, say, eight weaksauce minions in the way, hitting all of them, then you get another full action. You flurry to... either get an extra attack or hit each minion once, I'm not sure how it works, but you also cleave them, so you've *probably* taken down eight minions, and get a grand total of 12 attacks on the boss at minimum, more if he's in cleave range of the minions and you decide to hit them, and even more if you can flurry + charge for two attacks on each creature.

You have large size, a reach weapon, and you're surrounded by foes. You whirlwind attack, cleave, recharge, and whirlwind attack again... which, by being surrounded by 24 foes, means you get 96 attacks off, with 2 to each foe and 48 to distribute to taste. If you can flurry whirlwind attack, because, again, attack actions are poorly defined... that's 192 attacks.

You don't have your ultra powerful one per encounter double my doubled attacks capstone ready, so you just make clones of yourself. Every round. Or you steal attacks from enemies for two more attacks yourself, every round. So you've either got 10 attacks and an extra mirror image each round just by full attacking while moving normally (but very fast), or you've got 12 attacks and an unsaveable but confusing status effect on the enemy (does losing an attack mean attack roll using spells fail for the first one? Is it their highest or lowest BAB attack? I don't know).

All this, wrapped up with all good saves, d8 hit die, great movement abilities that also work as a defense, and the ability to autorevive with true res if they die somehow.