Urban Stalker

Urban Stalkers are beasts that come from the depths of the Mad God's realms. They would make lairs in cities and remain undetected as they feed upon those within the city. Since no one except for children and the victims could see them, they were often dismissed as fanciful tales. And considering that they could see the Urban Stalkers, many hunted children in order to keep their presence secret.
After the Mad God was imprisoned by the other Old Gods, Urban Stalkers all but disappeared from the Isles. That is, until the Bookhouse started to weaken the restraints that kept the Mad God locked up and creatures of all types began to reemerge from the nether realms.

Urban Stalker CR: 10

Large aberration, chaotic evil

Armor Class: 17

Hit Points: 135 (18d10+36)

Speed: 30 ft

STR

21 +5

DEX

9 -1

CON

15 +2

INT

18 +4

WIS

15 +2

CHA

18 +4

Saving Throws: CON +6, INT +8, WIS +6

Skills: Perception +10

Senses: Darkvision 120 ft., Passive Perception 20

Challenge Rating: 10

Proficiency Bonus: +4

Invisibility. The Urban Stalker is invisible to all creatures except for children, the mad, and anyone it attacks. When the Urban Stalker attacks a character, that character can see the creature until the Stalker's next action.

Actions

Multiattack. The Urban Stalker makes two tentacle attacks.
Tentacle. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 12 (2d6+5) bludgeoning damage.
Bite. Melee Weapon Attack: +9 to hit, reach 5 ft. one target. Hit: 15 (3d6 + 5) piercing damage, and the target is grappled (escape DC 14). Until this grapple ends, the target is restrained, and the Urban Stalker can't bite another target.
Swallow. The Urban Stalker makes one bite attack against a Medium or smaller target it is grappling. If the attack hits, the target is swallowed, and the grapple ends. The swallowed target is blinded and restrained, it has total cover against attacks and other effects outside the Urban Stalker. The Urban Stalker can have only one target swallowed at a time. If the Urban Stalker dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 5 feet of movement, exiting prone.

Legendary Actions

The Urban Stalker can take 4 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 Urban Stalker regains spent legendary actions at the start of its turn.
Bite. The Urban Stalker may use one Legendary Action to make one bite attack.
Swallow. The Urban Stalker may use one Legendary Action to make one swallow attack.
Digest (Costs 2 Actions). One creature swallowed by the Urban Stalker takes 10 (3d6) acid damage, and the Urban Stalker regains hit points equal to the damage the creature takes.
Move. The Urban Stalker can use a legendary action to move up to its movement rate without provoking an attack of opportunity from anyone who cannot see it.