Armor Class 21 (natural armor)
Hit Points 333 (19d20+133)
Speed 40 ft., fly 80 ft., swim 60 ft., climb 40 ft.
Damage Immunities acid
Condition Immunities charmed, exhaustion, frightened, paralyzed
Senses blindsight 60 ft., darkvision 120 ft., tremorsense 60 ft. (water and swamp terrain), passive Perception 22
Languages Common, Draconic, Sacred Magi, and all mortal languages spoken in the age of Venn
Challenge 21 (33,000 XP)
Legendary Resistance (3/Day). If Morthak fails a saving throw, it can choose to succeed instead.
Magic Resistance. Morthak has advantage on saving throws against spells and other magical effects.
God-Forged Constitution. Morthak was made to fight beings of near-divine power. It has advantage on saving throws against divine spells and the abilities of celestials and fiends.
Amphibious. Morthak can breathe both air and water.
Swamp Sovereign. Morthak cannot be tracked in swamp, marsh, or wetland terrain by any nonmagical means. Magical tracking requires Morthak to fail a DC 22 Wisdom saving throw. Additionally, Morthak has advantage on all Dexterity (Stealth) checks made in wetland terrain.
Scrying Ward. The Nexus's ambient magical saturation, combined with Morthak's presence, makes scrying within 5 miles of its current position automatically fail for any caster of less than 20th level.
Lore
Morthak chose a name meaning what the water covers and does not return. It chose this name for the same reason it chose the Nexus: because the things that disappear into deep water stay disappeared, and Morthak has always found that useful.
The Black Vennite Dragon was the one that operated where the other four could not -- in the swamps, the channels, the places where the ground was not ground and the water was not water and nothing moved in a straight line. During the Spirit Dragon Wars, this meant Morthak operated in the territories that the Spirit Dragons considered too disordered to govern, which were consequently the territories least watched. From those territories, Morthak struck supply lines, disrupted communications, and collapsed the logistical infrastructure that the Spirit Dragons' campaign of oversight depended on. It did not fight battles. It made battles impossible to sustain.
When the Wars ended, Morthak disappeared into the Nexus, which is perhaps the one place in Dracomere where a gargantuan dragon can remain genuinely hidden if it wishes to. The magical saturation of the fractured reality disrupts scrying. The terrain resists mapping. Whatever lives in the deepest channels of the Nexus does not surface for things that do not require surfacing.
There is an additional reason Morthak chose the Nexus specifically, one that it has not shared with any living creature: it knows what is beneath reality's fracture. It has known since the Wars, when something in the deep water spoke to it briefly and then went silent. It has been keeping watch over that silence ever since, not because it was instructed to, but because it concluded that something that powerful and that quiet warranted watching.
Habitat
Unknown; the Nexus contains zones of unusual magical saturation that the Orders of Wizardry classify as residual energy from the Great Sundering. Scholars who have studied these zones carefully have noted that some of the saturation patterns are not consistent with the Sundering's documented energy signature. No one has published this observation.