So I want to give my marshal classes in my game to have that feel of fighting multiple does with awesome attacks. 

Things like a barbarian swinging his club to smack two or more goons that ganged up on him, or an archer firing three arrows at once to hit multiple targets that are grouped together. 

So looking at the hunter ranger subclass level 11 ability, I want to give the marshals similar if slightly watered down abilities. 

So here are two ideas for how to make this work. 

For melee attacks

As an action the attacker can make an attack on two or more targets within five feet. 

A The penalty is either -1 to Attack rolls per additional target (can hit a number of enemies equal to your modifier)

Or 

B Penalty is -1 to damage rolls per target attacked. (Eg attack three targets, each attack losses 3 points of damage)

For ranged attacks like arrows and guns it's similar to hunters volley ability. Pick a point and make an attack on all targets within ten feet of that spot but apply penalty A or B 

So what are your thoughts? To O.p? to U.p? Any advice or opinions would be nice. I just want my marshals to feel like they can take on groups of enemies like badasses. Sometimes it's hard to feel strong when you take out goblins one at a time when in one turn a wizard just nukes the field with fireball and sweeps the encounter. 

For Barbarians I'd just make the penalty disadvantage on the attacks; this forces them to use Reckless Attack as a trade off, which I think is fine and perfectly in character.

Fighters are more difficult, as with Action Surge they have access to four attacks in a turn from 5th level (5 with two-weapon fighting) and zero penalties, and from 11th they can fight three enemies every turn if they want to. Arguably they don't really need a feature like this.

There actually is an optional rule for a cleave attack in the dmg (I found out yesterday.) I'll link it here.

Yeah I think the official cleave rule is an easy thing to add without really messing with the action economy. You could also simply split the damage - attack still does the same, but if it targets two creatures, each take half the damage.

Keep in mind that the Hunter 11 feature is quite strong, and partially balanced by the idea that it puts the Ranger - not the most durable class - in a precarious position. It would be quite a bit less risky to have on a fighter or barbarian, and would boost them quite a bit, especially just giving it to them for free.

Two things about martial attacks vs fireballs:

• Martial attacks work all day long while fireballs are limited. A long adventuring day is the easiest way to make martials shine
• To an extent, fireball is just supposed to be more powerful. A level 20 fighter will never be able to compete with meteor swarm. That is an intentional design choice in 5e.