We all know that a monk can hit up to 4 times per round, so I'm trying to compile ideas for multiclass paths that include things to add damage to each of those hits.  

This is inspired by my monk with a few levels of warlock.  I hex people, then hit them twice, adding 2d6 to the attack.  Then on the next turn, it really pays off as I can add 4d6 to the damage, on top of the damage they take from my weapon and/or fists.  

I'm avoiding things that add the damage to ONE hit per turn; as an example my above character took the Gene as his patron so I can add genes wrath damage to ONE of the four attacks I make.  I guess that's cool, but not as cool as adding damage to EACH of the attack's I make.  Probably still worth discussing though.      

So far, I can think of Hex and spirit shroud for spells.   

I think Divine Fury from a Zealot Barbarian maybe does this?  I cant really tell from the description.  

Dueling Fighting style from Fighters adds a bit as well.  

What else can you think of? 

Hunter's Mark, it'll be the same as Hex as far as damage goes but you'd get basic tracking on the creature for an hour.

The downside of Hunter's Mark is it specifies 'weapon attack', but with TCoE, that shouldn't be an issue with Ki-Fueled attack.

You could also grab Hunter's Mark or Hex with the Fey Touched feat instead of level dipping into Warlock or Ranger.

 For the barbarian comment: Divine Fury only does 1/turn not per attack.

In the magic items department, I can think of 3 that will boost your unarmed strikes.

The Insignia of Claws gives you a +1 on attack and damage

The Eldritch Claw Tattoo does the same plus 1d6 per attack and a 15 feet range (those two parts require a bonus action to activate so beware of the start-up lag if combined with Hex)

The Gloves of Soul Catching are probably the best magic item for a monk period.

Oh no, you can't be qualified to advise on Monk builds if you don't recognize that unarmed strikes are weapon attacks. ;)

See if a buddy can cast Crusader's Mantle. Or I guess you could get Divine Favor somehow. Or both!

Any monk weapon with +1 to +3 will work. Try to get one that does more than your martial arts dice. A quarterstaff or great club are good all the way through 16th level.

With the optional monk features from TCoE a standard longsword wielded 2H works through tier 4.

Quote from Zehal >>

 

The Gloves of Soul Catching are probably the best magic item for a monk period.

 Just looked these up, how is that even fair lol.  

Quote from Discfan2>>

The downside of Hunter's Mark is it specifies 'weapon attack', but with TCoE, that shouldn't be an issue with Ki-Fueled attack.

Actually this is fine; unarmed strikes are weapon attacks, your fists and feet etc. are however not weapons. So spells like Divine Favor and Hunter's Mark work just fine with all basic Monk attacks, but Branding Smite and Elemental Weapon do not work with unarmed strikes as they specify "your weapon".

They're Legendary, at this point it's not about being fair but about being as amazingly broken as possible. And honestly, a high level monk can really use the help in the damage department

A legendary item that "only" boosts your combat effectiveness is hardly the wildest thing in the Legendary tier of magic items, lol. Still, any Monk would gladly fight a dragon to get a pair of those gloves.

You are limited to a monk weapons but you're not giving up your bonus attack with Ki-Fueled attack. You are right about giving up FoB's 4th attack. But the point of hunters mark was more the tracking vs disadv on an ability check. So campaign dependent and what you're wanting.

If you're bringing in odd racial abilities then don't forget half orcs' savage attack.  It is only crits, but it is every weapon crit in a turn.

The problem with Hex/Hunter's Mark is that although you get an extra 2d6 on round 1, that damage comes instead of your bonus action attack so you don't really benefit unless

1. the target survives to your second round
2. the target has enough hp left that you couldn't have killed them without that extra damage

In my experience, a lot of enemies simply don't last that long.

In addition to Insignia of Claws and the Eldritch Claw Tattoo, the Giant Belt line is also very good for raising your base damage. Setting your STR to 29 is equivalent to all your attacks using a +4 weapon and it can stack with actual weapon enhancements. The other great item I don't see mentioned here is the Ioun Stone of Mastery - +1 proficiency does a lot for monks, although it doesn't affect damage beyond increasing your hit chance.