Blackgate Behemoth

Gargantuan aberration (alien, mythos), neutral evil

Armor Class 16 (natural armor)
Hit Points 625 (25d20 + 125)
Speed 50 ft., climb 30 ft.

23 (+6) 12 (+1) 21 (+5) 16 (+3) 17 (+3) 21 (+5)

Saving Throws Wis +8
Senses passive Perception 13
Languages Aklo
Challenge 16 (15,000 XP)

Special Traits

  • Innate Spellcasting. The blackgate behemoth’s innate spellcasting ability is Charisma (spell save DC 19). It can innately cast the following spells, requiring no components:
    • At will: air walk, blight, diminish plants (as plant growth, but in reverse), meld into stone, mold earth, stone shape, stone tell (this spell works as clairvoyance except the sensor is any rock or stone the behemoth chooses. The stone can recall up to 24 hours of information)
    • 3/day each: flesh to stone, song of discord (as a fireball but does thunder damage), stone to flesh (as flesh to stone, but in reverse), wall of stone
    • 1/day each: animal friendship (9th level), clashing rocks, earthquake, symbol of insanity (my cause the symbol to appear in midair and the blackgate behemoth is immune to the spell effects), move earth, rampart
  • Communion of the Black Gate (1/Week). A blackgate behemoth can disgorge a mystical slab of stone from its inward parts, appearing as a closed door of weathered black stone one foot thick and up to 15 feet square, covered in gruesome alien script. Each 5-foot section of the Black Gate has a damage threshold of 12 and 180 hit points; if the Black Gate is destroyed, the behemoth has disadvantage on all d20 rolls for 24 hours due to the psychic shock and feedback, and it cannot create a new Black Gate until after this time. The door cannot be created in any location that is being directly observed by a creature whose Intelligence exceeds 2, save for creatures with the mythos subtype (or anything the GM deems appropriate, for example, other aberrations). Once the Black Gate is created, it endures for 1d7+7 days; a behemoth may have only one Black Gate in existence at a time.

    A blackgate behemoth uses this mystic portal to observe and sometimes communicate with creatures of the mortal world. The behemoth can observe the door and its surroundings at will as if using clairvoyance. It may also use detect thoughts, emanating from any point on the Black Gate, regardless of its distance from the door.

    The script graven upon the Black Gate can be deciphered with a DC 16 (Intelligence) Arcana or Religion check or comprehend languages spell. Failing the skill check by 5 or succeeding the check by more than 5 results in insanity unless the target succeeds on a DC 16 Wisdom saving throw. The target remains insane until it passes the saving throw which it may repeat after every long rest. While the target is insane it acts as outlined by the confusion spell. Creatures using magic to read the script are also subjected to this insanity. A creature not driven mad may read from the Black Gate for 24 hours, deriving answers to 1d6 questions as if casting contact other plane. A character studying the Black Gate gains advantage on any checks related to the divination school of magic, as long as the spell is cast while reading from the Black Gate. A creature making a knowledge related Intelligence skill check while reading from the door adds 1d6 to the result of the skill check (2d6 if the check pertains to creatures, objects, places, or effects related to the alien mythos).

    While a creature is reading from the Black Gate, once a day, the behemoth may draw a creature into the Black Gate, where it is trapped within a bizarre subspace realm that functions as maze. In addition, when the creature finally escapes from the maze, the behemoth may inflict one effect upon a trapped creature; the effects it may choose depend on how long the creature remained trapped within the Black Gate.

    Trapped Time Possible Effects 1 round charm person 2 rounds touch of idiocy (as feeblemind but the duration is 1 day) 3 rounds suggestion 4 rounds modify memory 5 rounds dream 1 minute geas

    Creatures drawn into a Black Gate, even for a single round, receive a blackgate brand.

  • Blackgate Brand (1/Day). A blackgate behemoth may inscribe the blackgate brand on the forehead or chest of a helpless creature. This mark, which often resembles a deformed black goat�s head, lasts for 14 days and cannot be perceived by the creature on which it is inscribed other than via true seeing. However, other creatures with Intelligence greater than 2 perceive it subliminally when they spend at least 1 minute within 10 feet of the creature. Such creatures are compelled as a geas (spell save DC 19) to seek out the blackgate behemoth or a Black Gate. Creatures of CR (or level) 7 or higher gain a +1d4 bonus on their saving throw. The effects last up to 2d6 days, though compelled creatures can attempt a new saving throw every 24 hours to remove the compulsion, with the DC decreasing by 1 each day after the first.
  • Divine Disruption. A blackgate behemoth weakens the local connection between divine spellcasters and their patrons. A divine spellcaster includes clerics and paladins as well as any warlock whose patron is not from the mythos. Creatures attempting to cast a divine spell within 60 feet of a behemoth must succeed at a DC 16 spellcasting check (as if making a spell attack roll) or the spell fails. Even if this check succeeds, a behemoth may expend one use of its legendary actions as a reaction to force a creature casting the divine spell to make two separate DC 16 Wisdom saving throws. If either save fails, the divine spellcaster�s spell is ruined. If both saves fail, the divine spellcaster is also stunned until the end of its next round by the psychospiritual backlash. This use of this trait can also interdict a creature�s supernatural class abilities granted through a divine spellcasting class, such as a paladin�s lay on hands, or a cleric�s domain powers or ability to channel energy.
  • Frictionless. The blackgate behemoth is under a constant freedom of movement spell.
  • Hibernation. A blackgate behemoth can enter a state of hibernation at will; doing so takes 1 minute. While in this state, it can take no actions and is incapacitated, as if it were in a deep sleep. However, the duration of its Black Gate and any spell-like ability it has used on itself are extended as long as it remains hibernating, as if no time is passing. A behemoth can remain in hibernation for as long as it wishes – while in this state, it does not need to eat or drink, nor does it age. Time effectively stands still for a hibernating behemoth. If it is jostled or damaged while hibernating, or if a creature touches its Black Gate or attempts to decipher the writing on it, the behemoth can attempt a DC 16 Wisdom saving throw. If it succeeds, it awakens in 2d4 rounds. Otherwise, it takes 1d4 days to awaken from hibernation. A blackgate behemoth can set the length of its hibernation when it first enters this state, so that it can awaken after a set amount of time has passed. When awakening at a set time in this manner, a behemoth needs only 1d3 rounds to rouse itself, with no Wisdom saving throw necessary.
  • Legendary Resistance (3/Day). If the blackgate behemoth fails a saving throw, it can choose to succeed instead.
  • Magic Weapons. A blackgate behemoth�s weapon attacks are magical.
  • Psychotic Paralysis. A poisoned creature reduced to 0 hit points by a blackgate behemoth�s attacks is driven mad as the neurotoxic effluents of its bite attack its brain. Each round that a creature remains unconscious while poisoned, it must succeed on a DC 16 Wisdom saving throw or gain a random form of madness � roll randomly to determine the length of madness � short, medium or long then and then roll on the appropriate table. Multiple failed saves result in multiple forms of insanity. Creatures driven insane in this fashion, and who remain unhealed, often become obsessed with the place where they became insane and may establish mystery cults devoted to calling the blackgate behemoth back to that place and sharing its gift of madness with others.
  • Speak with Beasts and Plants. The blackgate behemoth can communicate with beasts and plants as if they shared a language.
  • Under the Darkling Moon. When outdoors at night, the save DC for any of a behemoth�s abilities that requires a Wisdom or Intelligence saving throw is increased by 1; during the three nights of the new moon, the DC is instead increased by 3. During the nights of the new moon, if a blackgate behemoth using its aberrant vomitus ability to create a monster expends 3 legendary actions, the creature does not perish after one hour.


  • Multiattack. The blackgate behemoth uses Alien Prescence then makes four attacks of its choice.
  • Alien Presence. Each creature of the blackgate behemoth�s choice that is within 120 feet of it and aware of it must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute. While frightened the creature is confused (as the spell). 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 blackgate behemoth�s Alien Presence for the next 24 hours.
  • Bites. Melee Weapon Attack: +11 to hit, reach 10 ft., one creature. Hit: 27 (6d6 + 6) piercing damage and the target is poisoned for 1 day on a failed DC 16 Constitution saving throw (see psychotic paralysis for additional effects).
  • Tentacles. Melee Weapon Attack: +11 to hit, reach 20 ft., one target. Hit: 33 (6d8 + 6) bludgeoning damage. If the target is a creature, it must succeed on a DC 19 Strength saving throw or be knocked prone. Alternatively, the blackagate behemoth may choose to grapple the target (escape DC 16). Until this grapple ends, the target is restrained, and the blackgate behemoth losses one of its four tentacle attack options. While grappled the target automatically takes 33 (6d8 + 6) bludgeoning damage at the start of the backgate behemoth’s turn due to being constricted.

Legendary Actions

The blackgate behemoth can take 3 legendary actions, choosing from the options below. Only one legendary option can be used at a time and only at the end of another creature’s turn. The blackgate behemoth regains spent legendary actions at the start of its turn.

  • Tentacle. The blackgate behemoth makes one tentacles attack.
  • Seductive Compulsions. A blackgate behemoth may use this legendary action whenever it uses an enchantment effect (including its psychotic paralysis), forcing the target to save with disadvantage.
  • Aberrant Vomitus (Costs 2 Actions). The blackgate behemoth can spew out a heap of noxious multicolored tissue and bile that functions as a maximum hit point gibbering mouther under the behemoth�s control. At the GM�s option, it may use this ability to create any aberration of CR 6 or lower. This creature perishes 1 hour after its creation.


Blackgate behemoths are titanic, solitary alien beings native to the dark places between and beyond the stars, seeded by greater masters onto planets they might explore and enslave.

They are towering creatures, their bulk heaving with terrifying swiftness upon four clawed legs like tree trunks. Mismatched eyeballs glare from every part of its body, scattered among a host of gnashing maws and a veritable forest of lashing fanged tentacles. They may lurk in deep caves for decades at a time but emerge at times of cosmic conjunctions that only they can sense to deposit mysterious black monoliths across the landscape. It is these strange runecarved portals that give blackgate behemoths their name. A blackgate behemoth is over 30 feet tall and weighs 15 tons.

Cult Master. While their Black Gates undoubtedly serve some inscrutable cosmic purpose, behemoths also use them as lures for the curious or the foolish, drawing them in and using them to establish cultic cells. They encourage the creation not only of cults that bring them sacrifices and offerings to leave at their mysterious Black Gates but also to leave and create sleeper cells elsewhere. Those who unravel the mysteries of a Black Gate may be drawn into it by the behemoth lurking nearby, often using its meld into stone ability to listen to the pathetic paeans of praise offered up by its deranged cultists while reading their minds itself and opening their minds to the touch of its darkling masters far off in the deeps of space. Blackgate cults are typically led by a bard, sorcerer, or witch of 7th-9th level, while the cultists are typically 3rd-level aristocrats, commoners, experts, or warriors.

The alien intelligence of a blackgate behemoth is often strange to terrestrial sentients, as the uses to which they put their cults may appear nonsensical. Some simply enjoy the adulation of their faux divinity, but other behemoths develop curious obsessions and drive their cultists to make them a reality, building bizarre monuments, retrieving incomprehensible combinations of objects for ineffectual or debasing rituals, slaughtering all creatures of a certain kind, or driving out rival religions. Some direct their cultists to kidnap priests of all kinds and drag them before the behemoth to demonstrate the impotence of their faith in the face of its primeval interdiction of divinity, the better to convince them to abandon their calling and worship the behemoth or its starry masters.

Awakened Hunger. Blackgate behemoths have very little impact on the natural world when hibernating, but when they awaken they can cause massive and terrifying disruptions to the landscape and the creatures dwelling in it. They sap the living essence of vegetation with their diminish plants and blight abilities, drawing a strange sustenance from it even as they leave swaths of blasted and defoliated wastelands in their wake. A behemoth�s mere presence drives animals into a mad frenzy of wild slaughter, though by attuning its will it can turn their crazed behavior into calm compulsion as its eerily silent slaves. Blackgate behemoths are omnivorous, eating as a supplement to their biospheric draining, but they are as likely to bite a creature once to inflict its poison as to devour a victim entirely. They frequently create their own food using stone to flesh, and a behemoth�s lair is often marked with disgusting mounds of rancid, half-eaten flesh of its own creation.

Section 15: Copyright Notice

Mythos Monsters (5E) © 2020, Legendary Games; Authors Michael "solomani" Mifsud, Robert J. Grady, Mark Hart, Jeff Ibach, Alex Riggs, Scott D. Young, Jeff Lee, Matt Kimmel, and Jason Nelson.

This is not the complete section 15 entry - see the full license for this page