What has the most HP in D&D?

The tarrasque is notorious as the creature with the most hit points of all those featured in the Monster Manual (676 HP). The only other official monster with a challenge rating of 30 is Tiamat, the evil dragon goddess. The tarrasque is immune to fire, poison, and non-magical bludgeoning, piercing, or slashing damage. 27-Oct-2018

https://www.sideshow.com/blog/10-great-dd-monsters-that-arent-dragons/#:~:text=The%20tarrasque%20is%20notorious%20as,%2C%20piercing%2C%20or%20slashing%20damage.