Every DM wants a giant badass monster to throw at the players, but 5e makes these kinds of encounters difficult to make challenging for the players. There are several reasons why that is but that isn’t what I want to focus on. Instead let’s talk about how to adjust these creatures to be feel more mythic!
In every JRPG ever there are creatures that you must defeat by taking out the extremities before you can lay into the beast, and the death of a limb doesn’t mean the death of the creature itself. This is easily done for legendary creatures like the Ancient Red dragon.
Segment the Creature into parts and allot Hit Points
The first step is to look at the creature’s actions and legendary actions and decide on how many body segments you want your creature to have. Then once you’ve decided on that divide the creature’s total hit points among the segments and notate which actions those limbs have. Finally determine which segment is the important segment which is vital for the creature’s survival. Here is how I broke up the Ancient Red Dragon.
Head & Body (Vital)
Actions: Bite, Frightful Presence, & Fire Breath
HP 60 HP each
Actions: 1 claw attack each
Actions: 2 Tail Attacks
Actions: Wing Attack
Every non-vital segment has the protect vitals reaction. This allows you to diffuse the damage the creature takes between the limbs.
Protect Vitals. As a reaction when a creature targets this creature’s vital segment with an attack or spell, this body segment redirects the attack to itself.
Running Mythic Segmented Creatures
Breaking the Ancient Red Dragon into different pieces has a TON of benefits. Players love to grapple, paralyze, poison, restrain, and stun these baddies rendering them completely useless while they beat them into the ground like a piñata. I know Legendary Resistances exist, but they are really meant to prevent your baddie from being rendered incompetent. (polymorphing the dragon into a chicken or feeble minding the archmage for example) However, if the poison condition ONLY applies to the body segment the poisoned dagger struck the entire encounter changes. Here are the rules to keep in mind when running a segmented foe.
Vital Segment. If the Vital Segment is subjected to a condition it effects all the other ones as well (If you stun someone’s head it’ll effect every other appendage).
Spells & Saving Throws. The creature is treated as a single creature when considering things like fireball. The DM divides the damage taken among the segments however they want.
Conditions. Conditions only effect the single segment and not the whole creature unless its the vital segment. Some segments are unaffected by certain conditions (an arm can’t be blinded and wings can’t be deafened) these effects do not effect the segment unless the vital segment is effected by it. If a segment is restrained it prevents the entire creature from moving, but only the restrained limb suffers the other negative effects of the condition.
Other Changes. Each segment rolls a separate initiative. The creature retains its Legendary Resistances and Lair Actions, but looses any Legendary Actions it had (they should be moved over to a body segment).