Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.
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: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 4 (1d8) necrotic damage.
Claw. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.
Tail. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.
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 16 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.
Sound of Death (Recharge 5–6). The dragon introduces all creatures within a 60-foot line that is 5 feet wide to the horrors of death. Each creature in that area must make a DC 18 Constitution saving throw, taking 45 (10d8) necrotic damage on a failed save, or half as much damage on a successful one.
The dragon 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 dragon regains spent legendary actions at the start of its turn.
Detect. The dragon makes a Wisdom (Perception) check.
Tail Attack. The dragon makes a tail attack.
Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 19 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.
Description
Onyx dragons are directly linked to death in every way. Despite this, they are not ruthless hunters and killers, and instead tend to find themselves merely lingering around sites of past deaths with a curious fascination. However, this fascination with death, coupled with a strong distaste for all things living, will lead them to kill any weary adventurer who enters their domains. Those who enjoy being alive are better off avoiding onyx dragons.
Midnight Black
Onyx dragon wyrmlings' scales are varying shades of reds and yellows upon hatching. As they age, onyx dragons' scales turn a bold black, though some do retain the reds and yellows from their childhood in small patches. The black of their scales makes them nearly impossible to see at night, lending a hand to their stealthy tendencies.
Stay Down
Onyx dragons have an extreme distaste for necromancy, as it takes away the dead things they cherish so immensely. They believe that death is an honor, and that those who die should stay down. For this reason, they will immediately kill any Undead they encounter and harbor intense hatred towards necromancers.
Although they do not actively attempt to die, this belief leads many of them to accept their deaths gracefully if they believe it is their destined end. If a necromancer revives an onyx dragon for any reason, they will instantly become extremely hostile and kill any creature they encounter until they are returned to rest.
Graveyard Hoards
Onyx dragons' obsession with death causes their hoards to closely resemble mass graves. They collect both the memorabilia of the dead and the dead themselves, littering their lairs with bones and artifacts belonging to those long dead.
Lair and Lair Actions
Onyx dragons most commonly dwell on or near ancient battlefields or mass graves. An onyx dragon’s lair is a ruin or graveyard where the stench of the dead lingers in the air. The lair is stained with blood or bones, and it is never quite clear if they are the dragon's victims or its decorations. Centipedes, rats, scorpions, and snakes infest the lair. The dragon takes comfort in the presence of dead creatures, so the litter of death always remains.
The challenge rating of a legendary onyx dragon increases by 1 when it's encountered in its lair.
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:
Beguiling Whisper. The dragon telepathically whispers to one creature within range of the dragon’s telepathy. The creature must succeed on a DC 15 Wisdom saving throw or be [Tooltip Not Found] by the dragon until initiative count 20 on the next round. A creature [Tooltip Not Found] in this way obeys to the best of its ability any command the dragon issues that isn’t directly harmful to the creature.
Death's Assistants. A swarm of skeletons raise from the ground in a 20-foot cube centered on a point the dragon chooses within 120 feet of it. The swarm spreads around corners and remains until the dragon dismisses it as an action, uses this lair action again, or dies. Any creature in the swarm's space when it appears must succeed on a DC 16 Constitution saving throw, taking 11 (2d8) slashing damage on a failed save, or half as much damage on a successful one. A creature is deafened while within the swarm's space.
Perpetual Darkness. Magical darkness spreads from a point the dragon chooses within 60 feet of it, filling a 15-foot-radius sphere until the dragon dismisses it as an action, uses this lair action again, or dies. The darkness spreads around corners. A creature with darkvision can’t see through this darkness, and nonmagical light can’t illuminate it. If any of the effect’s area overlaps with an area of light created by a spell of 2nd level or lower, the spell that created the light is dispelled.
Screams of the Dead. The screams of the dead fill the lair. Any creature within 5 feet of a grave must succeed on a DC 15 Constitution saving throw, taking 10 (3d6) psychic damage and becoming frightened on a failed save.
Regional Effects
The region containing a legendary onyx dragon’s lair is warped by the dragon’s magic, which creates one or more of the following effects:
Crystal Profusion. Natural stone within 6 miles of the lair grows plentiful crystal formations and veins of onyx gemstones, particularly underground.
Death Sight. Corpses within 6 miles of the lair become conduits for the dragon’s psionic presence. As an action, the dragon can cast the clairvoyance spell, requiring no spell components and targeting any corpses in that region.
Thriving Wildlife. Scavengers, such as vultures, coyotes, and hyenas, are attracted to the area within 6 miles of the lair and settle there in large numbers.
If the dragon dies, corpses remain where they were left, but other effects fade over 1d10 days.