I would say its at the DM’s discretion. However as one myself, i would describe it like if a PC spellcaster (specifically a wizard) wished to become a lich. They would need to learn and perform a ritual or process to become that.
note: this is my own opinion. Nothing in the rules as written specifically states that. However with options like the Reborn from Van Richtens or the Hollow One from Wildemount you could describe yourself as one.
Does an Oathbreaker turn into a Death Knight when they die?
According to the discription on what a Death Knight is in the Monster Manual they can. It's not that they do but can be.
If the DM wants them to. It’s definitely a DM discretion thing.
I would say its at the DM’s discretion. However as one myself, i would describe it like if a PC spellcaster (specifically a wizard) wished to become a lich. They would need to learn and perform a ritual or process to become that.
note: this is my own opinion. Nothing in the rules as written specifically states that. However with options like the Reborn from Van Richtens or the Hollow One from Wildemount you could describe yourself as one.
It really depends on your dm but In my experience only if they are powerful and or take steps to take the form of a death knight