Oathbreaker doesn't really have much combo potential with the Undead Warlock, other than the normal "paladin + warlock = short rest smite slots".
You might be thinking of the Oath of Conquest. That one gets an aura at level 7 which stops movement of frightened enemies. Then the Undead Warlock's form of dread lets you frighten an enemy each round with your attacks. So, I would get paladin to 7 ASAP, then take Warlock 1. The conquest paladin side gets important things at 8 (ASI), 9 (Fear spell), 10 (aura of courage), and 11 (improved divine smite), so I would push for those (and maybe the level 12 ASI too) before taking more Warlock levels. The nice thing is that Form of Dread only really cares about your proficiency bonus, so it can afford the wait.
When it comes to multiclassing, I'm the type who tends to pick one "main" class, and with the other I tend to just try to get the absolute minimum number of levels from that secondary class.
The simplest option is to take just one level of Warlock... you get the benefits of your Form of Dread from Undead right at first level, and you get a single spell slot that comes back on a short rest. You get more uses of your Form of Dread based on your Proficiency Bonus, not your Warlock level, so it still improves even if you're not putting levels into Warlock. The only thing you miss out on is the additional temp HP the feature grants, but that temp HP isn't as necessary if you're primarily a Paladin... a big boost of temp HP for a Warlock in light armor with a d8 Hit die, but for a Paladin who can just pop on some heavy armor and a shield, plus with a d10 hit die, they're already pretty good at surviving with or without temp HP.
I will say, though, that it might be worth it to get even just 1 more level of Warlock to get that second spell slot on a short rest. One of the problems with multiclassing Paladin and Warlock is the limited spell slot growth for both classes. But that's really only a problem if you're putting a lot of levels into Warlock... you're likely only ever going to get 2 spell slots at a time from this multiclass, so once you have both then, at the very least, you'll have two smites in the tank any time you take a short rest and still will make decent progress on your spell slot growth in Paladin. You also get two Eldritch Invocations at this stage... if your priority is combat, you could probably take Agonizing Blast and Repelling Blast just so you've got reliable damage and battlefield control even when you're out of melee range, or you could focus on more problem-solving features for out of combat. A fun combo is Mask of Many Faces and Beguiling Influence to get really good at infiltration. Could even pair well with the Actor feat if you need to round out your CHA score at some point.
I have a Paladin Oath breaker/Warlock undead patron what you think for levels of each or is there a build guide out there?
Oathbreaker doesn't really have much combo potential with the Undead Warlock, other than the normal "paladin + warlock = short rest smite slots".
You might be thinking of the Oath of Conquest. That one gets an aura at level 7 which stops movement of frightened enemies. Then the Undead Warlock's form of dread lets you frighten an enemy each round with your attacks. So, I would get paladin to 7 ASAP, then take Warlock 1. The conquest paladin side gets important things at 8 (ASI), 9 (Fear spell), 10 (aura of courage), and 11 (improved divine smite), so I would push for those (and maybe the level 12 ASI too) before taking more Warlock levels. The nice thing is that Form of Dread only really cares about your proficiency bonus, so it can afford the wait.
When it comes to multiclassing, I'm the type who tends to pick one "main" class, and with the other I tend to just try to get the absolute minimum number of levels from that secondary class.
The simplest option is to take just one level of Warlock... you get the benefits of your Form of Dread from Undead right at first level, and you get a single spell slot that comes back on a short rest. You get more uses of your Form of Dread based on your Proficiency Bonus, not your Warlock level, so it still improves even if you're not putting levels into Warlock. The only thing you miss out on is the additional temp HP the feature grants, but that temp HP isn't as necessary if you're primarily a Paladin... a big boost of temp HP for a Warlock in light armor with a d8 Hit die, but for a Paladin who can just pop on some heavy armor and a shield, plus with a d10 hit die, they're already pretty good at surviving with or without temp HP.
I will say, though, that it might be worth it to get even just 1 more level of Warlock to get that second spell slot on a short rest. One of the problems with multiclassing Paladin and Warlock is the limited spell slot growth for both classes. But that's really only a problem if you're putting a lot of levels into Warlock... you're likely only ever going to get 2 spell slots at a time from this multiclass, so once you have both then, at the very least, you'll have two smites in the tank any time you take a short rest and still will make decent progress on your spell slot growth in Paladin. You also get two Eldritch Invocations at this stage... if your priority is combat, you could probably take Agonizing Blast and Repelling Blast just so you've got reliable damage and battlefield control even when you're out of melee range, or you could focus on more problem-solving features for out of combat. A fun combo is Mask of Many Faces and Beguiling Influence to get really good at infiltration. Could even pair well with the Actor feat if you need to round out your CHA score at some point.
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