Boss Battle Botherations

[Fightmaster]

I'm trying to design a CR 11 legendary creature but I'm not sure how one calculates how legendary actions affect CR. Does anyone have any advice?

[pwykersotz]

I'm surprised at myself, I haven't broken down any creatures with Legendary Actions yet...

Without having done so to confirm, I believe Legendary Actions would be rolled into the existing metrics. So basically, take the maximum damage they can do in a round and roll it into the damage dealt over three rounds, then divide by three to figure out the offensive CR. If they duplicate abilities on page 280-281 in the DMG, follow the chart guideline, but only for the highest value that can be used during a turn. So if you give them Stench and Superior Invisibility, both usable for 3 Legendary Actions (max 3 per round), then raise the effective AC by 2, since Superior Invisibility raises it most but it can only do one at a time. If they're both usable in the same round, add them together for a +3 effective AC.

[RickAllison]

I'm surprised at myself, I haven't broken down any creatures with Legendary Actions yet...

Without having done so to confirm, I believe Legendary Actions would be rolled into the existing metrics. So basically, take the maximum damage they can do in a round and roll it into the damage dealt over three rounds, then divide by three to figure out the offensive CR. If they duplicate abilities on page 280-281 in the DMG, follow the chart guideline, but only for the highest value that can be used during a turn. So if you give them Stench and Superior Invisibility, both usable for 3 Legendary Actions (max 3 per round), then raise the effective AC by 2, since Superior Invisibility raises it most but it can only do one at a time. If they're both usable in the same round, add them together for a +3 effective AC.

This is an excellent point. If you would like more variable bonuses (i.e. switching between offense and defense), average them over three turns like the damage calculation.

[Ninja_Prawn]

I've tried calculating CRs for legendary creatures based on the 'maximum theoretic damage across three round' model, and I've found that it doesn't really work. Have a look at the ancient dragons in the MM - by that calculation they should be several points of CR higher than what they've actually been given.

If I needed to homebrew a huge boss, I'd use Angry's Paragon Monster rules instead of the legendary action / lair action stuff in the books. At least that way I can be confident that I'm getting the level of challenge I actually want.

[RickAllison]

I've tried calculating CRs for legendary creatures based on the 'maximum theoretic damage across three round' model, and I've found that it doesn't really work. Have a look at the ancient dragons in the MM - by that calculation they should be several points of CR higher than what they've actually been given.

If I needed to homebrew a huge boss, I'd use Angry's Paragon Monster rules instead of the legendary action / lair action stuff in the books. At least that way I can be confident that I'm getting the level of challenge I actually want.

How did you get them higher? When I calculate, I sometimes have difficulty getting the ancient dragons to their CR (and it usually is something like 23.5 rather than 24).

[Ninja_Prawn]

How did you get them higher? When I calculate, I sometimes have difficulty getting the ancient dragons to their CR (and it usually is something like 23.5 rather than 24).

Oh, maybe I got it the wrong way round. It was a long time ago when I was doing it... all I can remember is that they were way off. And it might have been the adults I ran the numbers on. Either way, I don't trust the DMG figures for anything above CR 12 or so.

[RickAllison]

Oh, maybe I got it the wrong way round. It was a long time ago when I was doing it... all I can remember is that they were way off. And it might have been the adults I ran the numbers on. Either way, I don't trust the DMG figures for anything above CR 12 or so.

When I calculate them per the DMG's guidelines with maximized damage from legendary attacks (3 tails), hitting two enemies with breath weapons (per guidelines), and using multiattack, and including pertinent adjustments for legendary resistance and saving throw bonuses, it usually works out within a half CR. Also remembering to include saving throw bonuses, but not Frightful Presence or the fly+ranged adjustments since they are expected to face higher-leveled parties.

EDIT: The damages I found for Ancient Brass Dragon, for instance, were 153/100/100 and the trait adjustments I used were +2 AC for the four saving throw bonuses and +90 HP for the 3/day Legendary Resistance. He ended up with a 21 defensive CR and a 19 offensive CR.

[Fightmaster]

I see, I think I've got the hang of it. However I'm wondering if granting a flat resistance to bludgeoning, piercing and slashing- magical or otherwise (the boss has the hide of a giant iron man-bat he beat) would be overkill. I haven't seen anything regarding resistance that doesn't care about magic. Should I consider it along the lines of an immunity and count it as X1.5 health?

[RickAllison]

I see, I think I've got the hang of it. However I'm wondering if granting a flat resistance to bludgeoning, piercing and slashing- magical or otherwise (the boss has the hide of a giant iron man-bat he beat) would be overkill. I haven't seen anything regarding resistance that doesn't care about magic. Should I consider it along the lines of an immunity and count it as X1.5 health?

They already have a thing for resistances! Same table as for immunities, just lesser modifiers. Now, you might go with immunity modifiers if the party doesn't really have ways around it (i.e. very few casters or other elemental damage), but normally resistance should be fine.