I understand what an oath breaker paladin is and the story and the player wants their paladin to break his oath and become an oathbreaker paladin but I don't know how to make his character an oath breaker on d&d beyond. Could someone tell me how?
This is something you need to discuss with your DM. Some may say you can not break an oath until after you have made it and so you can not start level 3 as an oath breaker. This however goes against the rules that you can not change you subclass, which would imply if you want to be an oath breaker you have to choose that at level 3.
TO make it on d&db I think you'd need the DMG otherwise the option wont be available.
Game wise: This really needs to be story driven, you'd have to have taken an oath and then willingly and knowingly broken it without making any attempt to rectify the problem.
An example could be the oath of devotion (Honesty, Courage, Compassion, Honor, Duty), if you ran from a fight to safe your own skin and let innoncents die/get sold into slavery then you would have failed n your oath od Courage, Honour and Duty and if you then lied about it then you have failed in your oath of Honesty and if you said they deserved it then you would also fail in your oath of compassion. BUT and there is a but....you also have to pursue some darker ambition such as letting your liege lord die in battle so you could take his wife and lands as your own becuase you also need to do it for your own self interest or to pursue some evil reason.
There isn't a lot of info in the DMG about Oathbreakers but the way I read it you also need to strike a bargain with a dark or evil power otherwise it doesn;t really make sense that a paladin gets their powers through their oath and then gets to keep/replace them by breaking it.
You could base a character off of the storyt of Lord Soth (although he became a death knight) or even Count Dracula (a Nobleman slowly warped by war to become a monster and abandons his holy intentions and takes up Vampirism to combat an invading force) but ultimately its going to be a DM call about whether you could actually do it. If you wanted to start at 3rd level alrready an Oathbreaker then you could reason it as you were a higher level paladin but the oath breaking caused your patron to stripe you of your power and vitality (effectively level draining you to level 2-3 or whatever level you are starting at) and your new patron has taken over but expect paladins and clerics of your old faith to come looking to end the life of your Oathbreaker.
I understand what an oath breaker paladin is and the story and the player wants their paladin to break his oath and become an oathbreaker paladin but I don't know how to make his character an oath breaker on d&d beyond. Could someone tell me how?
This is something you need to discuss with your DM. Some may say you can not break an oath until after you have made it and so you can not start level 3 as an oath breaker. This however goes against the rules that you can not change you subclass, which would imply if you want to be an oath breaker you have to choose that at level 3.
I think in dndbeyond there is no such option, I'm afraid...
TO make it on d&db I think you'd need the DMG otherwise the option wont be available.
Game wise: This really needs to be story driven, you'd have to have taken an oath and then willingly and knowingly broken it without making any attempt to rectify the problem.
An example could be the oath of devotion (Honesty, Courage, Compassion, Honor, Duty), if you ran from a fight to safe your own skin and let innoncents die/get sold into slavery then you would have failed n your oath od Courage, Honour and Duty and if you then lied about it then you have failed in your oath of Honesty and if you said they deserved it then you would also fail in your oath of compassion. BUT and there is a but....you also have to pursue some darker ambition such as letting your liege lord die in battle so you could take his wife and lands as your own becuase you also need to do it for your own self interest or to pursue some evil reason.
There isn't a lot of info in the DMG about Oathbreakers but the way I read it you also need to strike a bargain with a dark or evil power otherwise it doesn;t really make sense that a paladin gets their powers through their oath and then gets to keep/replace them by breaking it.
You could base a character off of the storyt of Lord Soth (although he became a death knight) or even Count Dracula (a Nobleman slowly warped by war to become a monster and abandons his holy intentions and takes up Vampirism to combat an invading force) but ultimately its going to be a DM call about whether you could actually do it. If you wanted to start at 3rd level alrready an Oathbreaker then you could reason it as you were a higher level paladin but the oath breaking caused your patron to stripe you of your power and vitality (effectively level draining you to level 2-3 or whatever level you are starting at) and your new patron has taken over but expect paladins and clerics of your old faith to come looking to end the life of your Oathbreaker.