Idea/Wishful thinking: The BBEG creator on DnDBeyond :)

I would love, if I could build my own legendary bosses, as easy as I can create played characters with the character builder. Take a monster and build on it as a template, selecting spells and legendary actions and so forth. In the end it would calculate XP, CR, treasure and math in the build so it was a fair but hard challenge.

My 3rd level party just ate my named goblin chieftainess Lady Gitbinn and her minions in 2 nova rounds, after much buildup, foreshadowing and an actual small city siege, that was very cool. She should have been a legendary monster, but I have no clue how to make one. How do you guys make your BBEgs and would you also want cool tools for creating legendary bosses?

One of the challenges of making a BBEG is how B the EG is (We know how B they are, because if they weren't B, the G guys wouldn't be after them!)

The Big Bad doesn't necessarily have to be big. Let's take a Moriarty-style enemy - they will be steps ahead of the party, leading them into traps, pitting them against minions and monsters, evading their clutches. When the party finally corners them, there shouldn't really be a fight - the Moriarty will have sent minions to fight for them, and if the party breaks through before they can escape, they will drop to a firm kick.

Then there's the actual Big Bad - the Balrog, or the Nemesis, where just fighting them is the challenge - realistically, the party may have been avoiding them until they are ready.

Here's how I do it - I haven't got a formula, but I do have a general approach.

1: Give them reactions, legendary actions and lair actions. Action economy is king - so having the boss stand around for a full round doing nothing is, frankly, just asking for them to get stunlocked and beat down with large, blunt instruments. As a rule, I think about what I want the boss to be doing, and make the legendary actions etc. thematic to that. Examples being:

• Huge stompy monster, legendary actions will include attacks, throwing things, biting/swallowing, and area of effects (EG jumping up and slamming down, sweeping with a club, that sort of thing). Reactions will be consequences, EG "on a missed melee attack against them, they can react to bite the attacker" style abilities.
• Masterminds, the legendary/lair actions will be to direct or spawn minions, cast spells, and reposition (EG teleporting, or just movement). They should be staying out of the fight, so moving away at the end of an enemies turn is a wise option. Reactions might include directing their minions to react, or some sort of emergency measure they hold (EG emergency plane shift).
• Experts, like sword masters and assassins and the like, will have highly themed ones - move and attack, grapple and throw, shoot and hide, react to disarm and attacker, Parry, that sort of thing. A Master's lair actions might see their training equipment come into play, launching arrows and fire bolts, or having the floor start to move - cool things like that.

When I make a creature, I use the Tetra-Cube creature creator, because it's convenient and spits out markdown code for homebrewery. It used to be more convenient, but now it's got more adverts than a late-1990's website, all of which try to pop up and make noise, which is honestly driving me away from it (if the owner of that site is listening, sort it out, please!). However, it does allow you to load an existing statblock and then change it. Once you're used to using it, it's really handy!

For CR, I will try to find something which is about equal to base it off - if I have a big monster that throws a big AOE of ice out, and can climb the walls, I might base it off a dragon (AOE ~= Breath Weapon, Climbing ~= Flying) and then adjust the hp to try and compensate - making it easier to kill if it's doing more than the base creature, and harder to kill if it's doing less (in the above example, if the AOE is the same damage and space as a breath weapon, I'd add some hitpoints as climbing is less than flying).

Definitely worth considering that a Boss Fight should be bigger than the Boss. The fight should be a situation, with minions or effects, perhaps with a ticking clock or secondary objective which has bad results if ignored - that sort of thing.

I hope that this helped - ask any questions you might have and I'll try to answer!

I think something like this is actually a really neat idea. 

Yes, when using the homebrew tools on the site you can just start with a monster stat block and then start adding and editing to your heart's content. 

But it would be nice to have a tool that would maybe allow you to select from options that already exist on monsters without having to just kind of search through all the monsters with Lair Actions or Legendary Actions to see what kinds of things are available and (ideally) tested for balance.

I get that some parts would be difficult (calculating CR isn't straightforward anyway), but I think a Creature Builder similar to the Character Builder could be very helpful. Select "Legendary", and then select "3 Legendary Actions". This then gives you a dropdown of all the legendary actions you have access to through the Basic Rules and any sourcebooks you own/have access to. You can then scroll through, see what is available, and select the ones that fit your creature. 

Want them to have Lair Actions? Select "Lair Actions" and then "2 Lair Actions" and you get a dropdown of all the Lair Actions you have access to through the basic rules and sourcebooks you have access to. Select the ones that fit your creature. 

Then you can edit those to change damage types or things like that. This would allow people who are first getting into homebrewing creatures a little bit more to go off of of what WotC already has in mind for balanced Actions, Bonus Actions, Reactions, etc. Dropdown lists galore!! Have filters like "Natural weapons" or "ranged weapons" for different actions. Select Multiattack and then how many attacks can be made with it. Etc.