Great Old One or Outer God? That’s not what we’re determining here with the Black Goat of the Woods with a Thousand Young and instead I’ve conjured some statistics for the dubiously named Shub-Niggurath!
The All-Mother only got mentioned by H.P. Lovecraft in incantations, expanded upon later under the ghost names he wrote for or inside of letters. As the Mother Goddess she bred with Yog-Sothoth to beget the evil twins Nug and Yeb, and she mated with Hastur bearing forth Ithaqua, J’Zaha, and Zhar--and maybe Yig too but it’s unclear if that’s a certainty. There’s also a strong association between Shub-Niggurath and a Black Goat but whether that’s an avatar, Satan, or Pan is for scholars to decide. End of story: she’s an evil fertility goddess that in some instances sides with humankind and receives more worship than anybody else (among humans and otherwise).
Design Notes: This is loosely based on the lesser spawn from Pathfinder #46: Wake of the Watcher with some original bits thrown in (my take on Shub-Niggurath’s Immortality, for instance, and Endless Spawn). Her actual CR cruises in at 28 by the DMG’s reckoning but I popped her up 2 because I figure the stream of extra monsters are a concern worth at least that much.
Shub-Niggurath
Colossal aberration (great old one or outer god), chaotic evil
Armor Class 27 (natural armor)
Hit Points 682 (35d20+315)
Speed 30 ft., fly 80 ft. (hover)
[TABLE="class: grid, width: 475"]
[TR]
[TD]
[TD]
[TD]
[TD]
[TD]
[TD]
[/TR]
[TR]
[TD]
[TD]
[TD]
[TD]
[TD]
[TD]
[/TR]
[/TABLE]
Skills Arcana +14, Insight +15, Nature +14, Religion +14, Stealth +13
Damage Resistances cold, necrotic; bludgeoning, piercing, and slashing from magical or cold iron weapons
Damage Immunities acid, fire, lightning, poison; bludgeoning, piercing, and slashing from nonmagical weapons not made from cold iron
Condition Immunities charmed, diseased, frightened, paralyzed, petrified, poisoned
Senses darkvision 120 ft., truesight, passive Perception 16
Languages Deep Speech; telepathy 300 ft.
Challenge 30 (155,000 XP)
Immortality. When Shub-Niggurath is slain, her form shrivels and compresses in on itself before exploding in a wave of milky fluid in a 200-foot radius. Any creature that comes into contact with the milky fluid makes a DC 25 Constitution saving throw. On a failed save, it is transformed into a random creature of an equal or lower CR, or a creature with a CR no higher than its level. A transformed creature makes a DC 24 Wisdom saving throw, keeping its intelligence and memories on a success. When all of the creatures transformed by the milky fluid have died, Shub-Niggurath is resurrected.
Innate Spellcasting. Shub-Niggurath’s innate spellcasting ability is Charisma (spell save DC 24; spell attack +16). She can innately cast the following spells, requiring no material components:
Insanity. Any creature that attempts to interact directly with Shub-Niggurath’s thoughts (such as via detect thoughts or telepathy) must succeed at DC 24 Wisdom saving throw or gain a long-term madness. When using her telepathy to communicate Shub-Niggurath doesn't activate this feature unless she spends an action to focus her mind on one opponent.
Legendary Resistance (3/Day). If Shub-Niggurath fails a saving throw, she can choose to succeed instead.
Magic Resistance. Shub-Niggurath has advantage on saving throws against spells and other magical effects.
Regeneration. Shub-Niggurath regains 20 hit points at the start of her turn if she has at least 1 hit point. Shub-Niggurath dies only if she starts her turn with 0 hit points.
ACTIONS
Multiattack. Shub-Niggurath can use her Endless Presence and Frightful Presence. She then makes seven attacks: one with her bite and six with her tentacles.
Bite. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 26 (4d8+8) piercing damage. If the target is a creature it is grappled (escape DC 25). Until this grapple ends, the creature is restrained, and Shub-Niggurath can't use her bite to grapple another target.
Tentacle. Melee Weapon Attack: +17 to hit, reach 25 ft., one target. Hit: 22 (4d6+8) bludgeoning damage. If the target is a creature it is grappled (escape DC 25). Until this grapple ends, the creature is restrained, and Shub-Niggurath can't use the tentacle to grapple another target (although Shub-Niggurath has an endless number of tentacles).
Endless Spawn (Recharge 4-6). Shub-Niggurath births creatures that she can mentally command. These creatures are loyal to her and defend Shub-Niggurath with their lives. Roll 1d4 to determine the creatures she births: on a 1 she creates 2d6 gibbering mouthers, on a 2 she creates 2d4 chuul, on a 3 she creates 1d6 otyughs, and on a 4 she creates either 1d4 cloakers (if underground) or 1d4 chaotic evil treants (if above ground).
Frightful Presence. Each creature of Shub-Niggurath’s choice that is within 120 feet of her and aware of her must succeed on a DC 24 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 Shub-Niggurath’s Frightful Presence for the next minute.
The All-Mother only got mentioned by H.P. Lovecraft in incantations, expanded upon later under the ghost names he wrote for or inside of letters. As the Mother Goddess she bred with Yog-Sothoth to beget the evil twins Nug and Yeb, and she mated with Hastur bearing forth Ithaqua, J’Zaha, and Zhar--and maybe Yig too but it’s unclear if that’s a certainty. There’s also a strong association between Shub-Niggurath and a Black Goat but whether that’s an avatar, Satan, or Pan is for scholars to decide. End of story: she’s an evil fertility goddess that in some instances sides with humankind and receives more worship than anybody else (among humans and otherwise).
Design Notes: This is loosely based on the lesser spawn from Pathfinder #46: Wake of the Watcher with some original bits thrown in (my take on Shub-Niggurath’s Immortality, for instance, and Endless Spawn). Her actual CR cruises in at 28 by the DMG’s reckoning but I popped her up 2 because I figure the stream of extra monsters are a concern worth at least that much.
Shub-Niggurath
Colossal aberration (great old one or outer god), chaotic evil
Armor Class 27 (natural armor)
Hit Points 682 (35d20+315)
Speed 30 ft., fly 80 ft. (hover)
[TABLE="class: grid, width: 475"]
[TR]
[TD]
STR
[/TD][TD]
DEX
[/TD][TD]
CON
[/TD][TD]
INT
[/TD][TD]
WIS
[/TD][TD]
CHA
[/TD][/TR]
[TR]
[TD]
27 (+8)
[/TD][TD]
18 (+4)
[/TD][TD]
28 (+9)
[/TD][TD]
21 (+5)
[/TD][TD]
23 (+6)
[/TD][TD]
24 (+7)
[/TD][/TR]
[/TABLE]
Skills Arcana +14, Insight +15, Nature +14, Religion +14, Stealth +13
Damage Resistances cold, necrotic; bludgeoning, piercing, and slashing from magical or cold iron weapons
Damage Immunities acid, fire, lightning, poison; bludgeoning, piercing, and slashing from nonmagical weapons not made from cold iron
Condition Immunities charmed, diseased, frightened, paralyzed, petrified, poisoned
Senses darkvision 120 ft., truesight, passive Perception 16
Languages Deep Speech; telepathy 300 ft.
Challenge 30 (155,000 XP)
Immortality. When Shub-Niggurath is slain, her form shrivels and compresses in on itself before exploding in a wave of milky fluid in a 200-foot radius. Any creature that comes into contact with the milky fluid makes a DC 25 Constitution saving throw. On a failed save, it is transformed into a random creature of an equal or lower CR, or a creature with a CR no higher than its level. A transformed creature makes a DC 24 Wisdom saving throw, keeping its intelligence and memories on a success. When all of the creatures transformed by the milky fluid have died, Shub-Niggurath is resurrected.
Innate Spellcasting. Shub-Niggurath’s innate spellcasting ability is Charisma (spell save DC 24; spell attack +16). She can innately cast the following spells, requiring no material components:
Constant: freedom of movement, true seeing
At will: black tentacles, dispel magic, dream, project image, sending
3/day: suggestion, feeblemind, symbol, weird
At will: black tentacles, dispel magic, dream, project image, sending
3/day: suggestion, feeblemind, symbol, weird
Insanity. Any creature that attempts to interact directly with Shub-Niggurath’s thoughts (such as via detect thoughts or telepathy) must succeed at DC 24 Wisdom saving throw or gain a long-term madness. When using her telepathy to communicate Shub-Niggurath doesn't activate this feature unless she spends an action to focus her mind on one opponent.
Legendary Resistance (3/Day). If Shub-Niggurath fails a saving throw, she can choose to succeed instead.
Magic Resistance. Shub-Niggurath has advantage on saving throws against spells and other magical effects.
Regeneration. Shub-Niggurath regains 20 hit points at the start of her turn if she has at least 1 hit point. Shub-Niggurath dies only if she starts her turn with 0 hit points.
ACTIONS
Multiattack. Shub-Niggurath can use her Endless Presence and Frightful Presence. She then makes seven attacks: one with her bite and six with her tentacles.
Bite. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 26 (4d8+8) piercing damage. If the target is a creature it is grappled (escape DC 25). Until this grapple ends, the creature is restrained, and Shub-Niggurath can't use her bite to grapple another target.
Tentacle. Melee Weapon Attack: +17 to hit, reach 25 ft., one target. Hit: 22 (4d6+8) bludgeoning damage. If the target is a creature it is grappled (escape DC 25). Until this grapple ends, the creature is restrained, and Shub-Niggurath can't use the tentacle to grapple another target (although Shub-Niggurath has an endless number of tentacles).
Endless Spawn (Recharge 4-6). Shub-Niggurath births creatures that she can mentally command. These creatures are loyal to her and defend Shub-Niggurath with their lives. Roll 1d4 to determine the creatures she births: on a 1 she creates 2d6 gibbering mouthers, on a 2 she creates 2d4 chuul, on a 3 she creates 1d6 otyughs, and on a 4 she creates either 1d4 cloakers (if underground) or 1d4 chaotic evil treants (if above ground).
Frightful Presence. Each creature of Shub-Niggurath’s choice that is within 120 feet of her and aware of her must succeed on a DC 24 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 Shub-Niggurath’s Frightful Presence for the next minute.
but i'll construct that in my lovecraft mythos campaign. 
