What is the most challenging monster in the Dungeons & Dragons 5th Edition Monster Manual?

The Tarrasque is widely considered to be the most challenging monster in the Dungeons & Dragons 5th edition Monster Manual. This colossal creature possesses an immense amount of hit points, extraordinary damage resistance, and multiple devastating attacks capable of annihilating entire parties of adventurers. Its legendary actions, including swallowing creatures whole and unleashing devastating tail sweeps, further add to its formidable nature. Its sheer size and overwhelming power make it a near-impossible foe to defeat, earning its reputation as one of the most challenging encounters in the game.