I want to create a monster with a resistance to nonmagical weapon attacks, that aren't made with cold iron weapons. DnD beyond won't let me because a weakness or resistance has to have appeared before. I find this to be completely inane. There are monsters in dnd 5e with highly specific vulnerabilties, resistances, or immunities, why can't I make my own in dnd-beyond? I don't see how this would be hard, just stop restricting the damage override to specific tags. Then we could put in whatever we wanted.

Beyond's main focus has always been implementing the official rules, so do not get your hopes up about homebrew improvements being a priority anytime soon, if ever. Beyond is not even able to implement every official rule yet, so stuff like homebrew, UA, and third party content are out of the question. The homebrew tools was originally built to implement official content when stuff was very standardized. It was not meant for the public in mind, and it certainly was not meant for homebrewing whatever users want.

With that out of the way, you CAN make custom vulnerabilities, even very specific and intricate ones, as you just put them in the abilities section.

The first section of the stat block is meant for tags for quick reference, so those fields are not very customizable. If I had to guess why it was implemented that way, I assume it is easier to select a limited number of tags rather than typing things in when they have to implement the official stuff, and it makes finding and sorting monsters easier.

Thanks for the quick response. Though I still think it is inane to include a specific vulnerability for Piercing damage dealt by weapons wielded by Good Creatures, as a tag. Just because it appears once in the books. And not include a way to add in new tags. It'd be different if they stuck to tags for general resistances or vulnerabilities, and moved the few exceptions to abilities, as you suggested.

Also, if the homebrew tool was not intended for public use. Then they shouldn't have released it for public use. But not only did they, they also encouraged it's use by allowing one to share their homebrew creations.

Quote from TheRoleplayer98 >>

Thanks for the quick response. Though I still think it is inane to include a specific vulnerability for Piercing damage dealt by weapons wielded by Good Creatures, as a tag. Just because it appears once in the books. And not include a way to add in new tags. It'd be different if they stuck to tags for general resistances or vulnerabilities, and moved the few exceptions to abilities, as you suggested.

Also, if the homebrew tool was not intended for public use. Then they shouldn't have released it for public use. But not only did they, they also encouraged it's use by allowing one to share their homebrew creations.

Wizards was against allowing homebrew at first. Beyond convinced them to allow some homebrew since there was great demand for it.