Xalthon (5e Creature)
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Large aberration (great old one), chaotic evil
Saving Throws Con+14, Wis+15, Cha+16
Legendary Resistance (2/day). If Xalthon fails a saving throw, it can choose to succeed instead.
Innate Spellcasting. Xalthon's innate spellcasting ability is Charisma (spell save DC 24). Xalthon can innately cast the following spells, requiring no material components:
2/day: warp dead
Eternal. When Xalthon is killed, its body does not decompose. There is no limit to how long after its death it can be the target of Revivify, Raise Dead, Reincarnate or Resurrect, those spells function on it even if it is undead, and if it is animated as an undead creature it controls itself.
Magic Resistance. Xalthon has advantage on saving throws against spells and other magical effects.
Regeneration. Xalthon regains 10 hit points at the start of its turn.
Destroy Plane. If every other creature on the same plane as Xalthon is a Minion of Xalthon, Xalthon can use his action to destroy the plane it's on (it cannot destroy the far realm using this feature). Xallthon and every creature on the same plane as it appear in a random plane (they all appear in the same plane, as tightly packed as is possible for them to each have their own square) and the plane they were previously on ceases to exist. If, as a result of this spell, a portal would connect to a plane that doesn't exist, that portal also ceases to exist. If a creature would be banished to its home plane but its home plane doesn't exist, then that creature disappears, and reforms 24 hours later in the same square it died in (or the closest unoccupied square if that square is occupied).
Warp Dead. Xalthon chooses up to four corpses of Medium or Small creatures within range. Each corpse becomes a Minion of Xalthon under its control, and all of its equipment is transformed into a +1 dagger which deals 1d6 extra psychic damage on a hit (if it has no equipment it doesn't get the dagger). Xalthon can use a bonus action to mentally command its Minions that are within 120 feet of it, issuing the same command to any or all of them. Xalthon decide what action the creature will take and where it will move during its next turn or issues a general command. The creature is under Xalthon's control for 48 hours, after which it dies. To keep the creature alive for another 48 hours, Xalthon must use this ability on the creature before the current 48-hour period ends. This use of the ability affects up to six creatures rather than animating new ones.
Multiattack. Xalthon makes two attacks with its knife, two grapple checks, or an attack with its knife and a grapple check.
Nightmare Vision. Recharge 6. Xalthon chooses a creature within 60 feet of it. The target is incapacitated until the start of Xalthon's next turn, at which time it takes 55 (10d10) psychic damage. Additionally, unless it has been less than a ten-day since the target had this feature used on it (by Xalthon) the vision, which it can't quite remember but knows was horrible, will come true within a ten-day unless the target dies before then (it is up to the DM how to resolve this, but Xalthon is balanced expecting the creature to take about 100 damage when the vision resolves).
Knife. Xalthon attacks with Creator of Thralls. Melee or Ranged Weapon Attack: +13 to hit, reach 10 ft. or 20/60 ft., one target. Hit: 8 (1d4+5) piercing damage plus 5 (1d8) psychic damage, and if the target is not an aberration it must make a DC 25 wisdom saving throw. On a failed save, until the knife is removed from its body the target doesn't know the knife exists, is charmed by Xalthon, and can communicate telepathically with Xalthon while the two are on the same plane of existence.
Insanity Hold. While Xalthon is grappling a creature, it may use his reaction at the start of that creature's turn to deal 28 (5d10) psychic damage to them and cast confusion on them.
The Xalthon can take 2 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 Xalthon regains spent legendary actions at the start of its turn.
Attack. Xalthon makes one knife attack.
A few of the creatures from the Far Realm are known, such as Azathoth, Yog-Sothoth and Cthulhu, but many more who have never before been known to the inhabitants of the Wheel. Xalthon is one of these creatures, an elder evil. Appearing as an 8 foot tall green humanoid with long skinny legs and 4 similarly long and skinny arms, it is able to inflict massive psychic damage with a glance, cause insanity with a touch, turn corpses into smaller replicas of itself, and even alter the future. Xalthon is always equipped with its magic dagger aptly named the "Creator of Thralls" after the similarly named warlock feature. It creates massive carnage, and the only creatures it won't kill without a thought are the great old one Taranokk and potential victims for Creator of Thralls.
As a Great Old One, Xalthon's long term goals can sometimes be strange, but it has intelligence 20 so they aren't incomprehensible, and its battle tactics are sound. Xalthon will usually attempt to make his army as big as possible, and destroy as many planes as he can, despite there being no obvious reward for destroying a plane. In combat Xalthon will target its enemies based on their ability to deal damage; it will ignore the enemy it thinks has the lowest Strength and Dexterity, and in case of a tie Wisdom, and it will target the remaining creatures from highest to lowest DPS using its Nightmare Vision when possible and its Eldritch Blast when not. Once there are only two enemies remaining Xalthon will attempt to grapple the creature with less Strength and Dexterity, and once they are grappled it will attempt to stab them. It will never throw its dagger. It will use its reaction for shield against the first possible attack unless it is grappling a creature; it will always use Insanity Hold if possible.