Alright, a little while ago I had nothing better to do than whip up a quick character, said character was a rogue 3/ ranger 3, taking assassin in rogue and hunter in ranger, with the horde breaker feature. He has the two weapon fighting style and has the feat mobile. All of this means that he can attack as many creatures as he wants (as long as they’re within five feet of each other), and unless I’m mistaken he can attack each twice, since two weapon fighting counts as one attack, with his ability modifier added to the damage. He also gets an crit on any creatures that are surprised and has advantage on any creatures that haven’t gone yet.  All while the monsters can’t use opportunity attacks against him. Please call me out on things I did wrong and overall tell me if this is a broken character build. Thanks!

Attacking requires burning an Action, which you only get one of per turn. Two weapon fighting and Horde breaker each give you one additional attack roll.

This allows you to hit 3 creatures at most, one time each. Or one creature twice and another once.

The important things you have missed are:

1: Horde Breaker says "once per turn..."

2: Two weapon fighting is "when you make an attack action...".

You need to tune your reading into the mechanics of the game. What you have is an Action (the thing you can do) and Abilities, which trigger when you perform the action or make an attack.

Two weapon fighting: "When you make an attack action with a light weapon, you can use a bonus action to make an attack with your off-hand" or something along those lines.

Hordebreaker: "Once on each of your turns when you make a weapon Attack..."

So, you do the following:

1: Have 2 weapons, one in each hand, both of which are light, to meet the requirements for two-weapon fighting
2: Make an attack action, using one hand to make the number of attacks you can make at your level (as you level up you will unlock Extra Attack in some classes to attack twice with one attack action).
3: Because you made an Attack Action, you can trigger your two abilities:
3.1: Hordebreaker, allowing you to make an extra attack against one different enemy within 5ft, immediately.
3.2: Two Weapon Fighting, allowing you to make another attack with your off-hand as a bonus action (you can move in between)

You can activate Hordebreaker after any melee weapon attack, meaning that you could use an Attack action, then move, then use your bonus action to offhand attack, then use Hordebreaker to make an additionaly attack.

Not broken, but not a bad build if that's what you're after!

Thanks guys, appreciate the feedback. Still getting used to the mechanics of the game so I will look into that. Again, thanks!

Quote from ThorukDuckSlayer >>

The important things you have missed are:

1: Horde Breaker says "once per turn..."

2: Two weapon fighting is "when you make an attack action...".

You need to tune your reading into the mechanics of the game. What you have is an Action (the thing you can do) and Abilities, which trigger when you perform the action or make an attack.

Two weapon fighting: "When you make an attack action with a light weapon, you can use a bonus action to make an attack with your off-hand" or something along those lines.

Hordebreaker: "Once on each of your turns when you make a weapon Attack..."

So, you do the following:

1: Have 2 weapons, one in each hand, both of which are light, to meet the requirements for two-weapon fighting
2: Make an attack action, using one hand to make the number of attacks you can make at your level (as you level up you will unlock Extra Attack in some classes to attack twice with one attack action).
3: Because you made an Attack Action, you can trigger your two abilities:
3.1: Hordebreaker, allowing you to make an extra attack against one different enemy within 5ft, immediately.
3.2: Two Weapon Fighting, allowing you to make another attack with your off-hand as a bonus action (you can move in between)

You can activate Hordebreaker after any melee weapon attack, meaning that you could use an Attack action, then move, then use your bonus action to offhand attack, then use Hordebreaker to make an additionaly attack.

Not broken, but not a bad build if that's what you're after!

Just need to correct something in your statement (3.2). You can move in between attacks if you have the Extra Attack ability, but if you're using a bonus action to make an off-hand attack, you can't move between the attack made with the primary weapon and the off-hand attack. The attack with the primary weapon is the trigger for the off-hand attack, the off-hand attack occurs immediately after the attack with the primary weapon.

If you have Extra Attack, you can break up your attacks and move in between them. However, if you choose to use a bonus action to use an off-hand attack after one of those primary weapon attacks, it must be done immediately after with nothing in between (such as movement).

Nothing in the rules for two weapon fighting says you can’t move after taking the attack action and before using your bonus action to attack with a second weapon. 

There is no restriction in the two-weapon fighting rules that specifies you can't move after taking the attack action and before using your bonus action to attack with a second weapon. 

bike race

We sent Jeremy Crawfod a question via Twitter and recieved a reply stating that the off-hand attack is different than multi-attack or extra attack, and that it was triggered by the primary attack and had to be taken immediately afterward with no move in between.

I have since deleted my Twitter account, but I just did a quick search online and found the following reply to someone else that says basically the same thing:  https://twitter.com/jeremyecrawford/status/1105170572273573888