Actions

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.   Bite. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: (2d10 + 10) piercing damage.   Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: (2d8 + 10) slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 28 (10+Athletics)) and is restrained until this grapple ends. The greatwyrm can only have one creature grappled this way at a time.   Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: (2d10 + 10) bludgeoning damage. If the target is a creature, it must make a DC 28 (8 +Prof + Str) Strength saving throw or be knocked prone   Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 24 (8 + Prof + Cha) Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.   Fire Breath (Recharge 5-6). The dragon exhales fire in a 300-foot cone. Each creature in that area must make a DC 26 (8 + Prof + Str) Dexterity saving throw, taking 99 (22d8) fire damage on a failed save, or half as much damage on a successful one.   Enraging Roar. The dragon roars as an action, every creature within 120ft that can hear the dragon must make DC 24 (8 + Prof + Cha) becoming Reckless on a failure. A creature who is Reckless must attack recklessly (as per the barbarian ability) and has advantage on physical saving throws but disadvantage on mental saving throws, these creatures can repeat the save at the end of each of their turns ending the effect on success. The dragon can use this action a number of times equal to it's proficiency bonus.   Change Shape. The dragon magically polymorphs into a Humanoid or beast that has a Challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any Equipment it is wearing or carrying is absorbed or borne by the new form (the dragon's choice). In a new form, the Dragon retains its Alignment, Hit Points, Hit Dice, ability to speak, Proficiencies, Legendary Resistance, lair Actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its Statistics and capabilities are otherwise replaced by those of the new form, except any Class Features or legendary Actions of that form.

Legendary Actions

The greatwyrm can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The greatwyrm regains spent legendary actions at the start of their turn.   Detect. The dragon makes a Wisdom (Perception) check.   Attack. The dragon makes a claw or tail attack.   Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 30 ft. of the dragon must succeed on a DC 26 (8 + Prof + Str) Dexterity saving throw or take 12 (2d6 + 5) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.   Ignite Blood (Costs 2 Actions). If the dragon is damaged it can take this action. The spaces the dragon has occupied or flown over since the start of it's last turn burst in flame as the dragon blood burns, any creature occupying those spaces must make a DC 26 (8 + Prof + Con) Dexterity saving throw, taking 12d6 fire damage on a failure or half as much on a success. A creature with Red Dragon Blood on it has to make the save even if it is just adjacent to an effected space, these creature also make the Dexterity saving throw at disadvantage. This fire ignites any flammable object that isn't being worn or carried, a object that has Red Dragon Blood on it is ignites if is adjacent to an effected space.  

Mythic Actions

If the greatwyrm's Draconic Awakening trait has activated in the last hour, it can use the options below as legendary actions.   Bite. The greatwyrm makes a bite attack.   Shattering Roar (Costs 2 Actions). The greatwyrm unleashes a magical roar. Each creature in a 120-foot-radius sphere centered on the greatwyrm must succeed on a DC 28 (8 + Prof + Cha) Constitution saving throw or take 19 (3d12) thunder damage and be incapacitated until the end of its next turn.   Enraging Roar (Costs 3 Actions). The greatwyrm uses its Enraging Roar action.

Lair Actions

On initiative count 20 (losing initiative ties), the dragon takes a lair action to cause one of the following effects; the dragon can’t use the same effect two rounds in a row:  

• Magma erupts from a point on the ground the dragon can see within 120 feet of it, creating a 20-foot-high, 5-foot-radius geyser. Each creature in the geyser’s area must make a DC 15 Dexterity saving throw, taking 21 (6d6) fire damage on a failed save, or half as much damage on a successful one.
• A tremor shakes the lair in a 60-foot radius around the dragon. Each creature other than the dragon on the ground in that area must succeed on a DC 15 Dexterity saving throw or be knocked prone.
• Volcanic gases form a cloud in a 20-foot-radius sphere centered on a point the dragon can see within 120 feet of it. The sphere spreads around corners, and its area is lightly obscured. It lasts until initiative count 20 on the next round. Each creature that starts its turn in the cloud must succeed on a DC 13 Constitution saving throw or be poisoned until the end of its turn. While poisoned in this way, a creature is incapacitated.

Red dragons lair in high mountains or hills, dwelling in caverns under snow-capped peaks, or within the deep halls of abandoned mines and dwarven strongholds. Caves with volcanic or geothermal activity are the most highly prized red dragon lairs, creating hazards that hinder intruders and letting searing heat and volcanic gases wash over a dragon as it sleeps.   With its hoard well protected deep within the lair, a red dragon spends as much of its time outside the mountain as in it. For a red dragon, the great heights of the world are the throne from which it can look out to survey all it controls—and the wider world it seeks to control.   Throughout the lair complex, servants erect monuments to the dragon’s power, telling the grim story of its life, the enemies it has slain, and the nations it has conquered.

Regional Effects

The region containing a legendary red dragon’s lair is warped by the dragon’s magic, which creates one or more of the following effects:  

• Small earthquakes are common within 6 miles of the dragon’s lair.
• Water sources within 1 mile of the lair are supernaturally warm and tainted by sulfur.
• Rocky fissures within 1 mile of the dragon’s lair form portals to the Elemental Plane of Fire, allowing creatures of elemental fire into the world to dwell nearby.

  If the dragon dies, these effects fade over the course of 1d10 days.