Cthulu

Natural Weapons. Cthulhu's natural weapons are based on strength

Eldritch Horror.  any creature who can see the creature goes insane and commits die. Cthulhu can only be seen with blindsight

Spellcasting. The [monster name] is a 20th-level spellcaster. Its spellcasting ability is wisdom (spell save DC 20, +5 to hit with spell attacks). The Cthulhu has following spells prepared:
Cantrips (at will): all
all levels (infinite slots): all

Tentacles. Cthulhu uses its tentacles take 10D10 bludgeoning damage and is grappled by Cthulhu. Cthulhu has 10 tentacles that can each grapple 1 target escape DC 20 

Swallow. Cthulhu swallows one creature grappled by him. escape DC15. if Cthulhu takes at least 60 damage from inside, he must make a DC 15 constitution saving throw or regurgitate all swallowed targets, which exit 10 ft away from Cthulhu  and exits prone

NO. veto an action