Running a game of Dungeons & Dragons takes experience and expertise that can be easy to underestimate. New Dungeon Masters may believe that because they command the game's rules and story, nothing is off-limits in terms of what they can do. While that is true to a degree, can and should are two different things.
As the ultimate arbiter of how a game works, making rulings and designing every encounter their party comes across, it is easy to quickly gain confidence and eagerness in running the game's biggest monsters once DMs know the basics of how to play. But the more rules, mechanics, and backstory that a monster entails, the more complicated they become to run. While the highest-level Challenge Rating monster, the Tarrasque, is actually pretty straightforward once a DM gets a knack for their basic attacks and immunities, any monster with spellcasting changes the game quickly.
Take Zariel, a demon from Mordenkainen's Tome of Foes, for example. At CR 26, she would seemingly be easier to run than a Tarrasque. However, she comes with a whole host of options that change how an encounter plays out significantly. On top of her immunities and resistances, Zariel has spells like Finger of Death and Wall of Fire that can instantly kill players unprepared for them, and which create minions and hazards DMs then have to track through a session. Meanwhile, they also have to track her Regeneration, Teleportation, and Legendary and Lair Actions that give her more than just a single turn to worry about in the action economy. Those last two points are perhaps one of the most complicated aspects for new DMs to keep track of.
Zariel can use a Legendary Action at the end of each player's turn to spontaneously combust a target she looks at, or a Lair Action to send out a Fireball. The most powerful monsters new DMs should steer clear of have similar options that are even more complicated. The demon Orcus can use Power Word Kill as a Lair Action to automatically kill players without even requiring a saving throw, and Demogorgon can use a Legendary Action to utilize its Gaze attack that comes with a variety of effects a DM has to constantly keep in mind. The strongest monsters a DM can run demand a mastery of spells, Legendary and Lair Actions, action economy, and condition effects that any new DM would find dizzying. And the most powerful monster of all is the worst: the Aspect of Tiamat.
The Aspect of Tiamat is simply too big for a new DM to run. While the Tarrasque may have the same CR rating, Tiamat is a far greater threat requiring the full range of the game's ruleset. Tiamat manifests as a five-headed dragon immune to almost all damage types, cannot roll almost any saving throw at anything less than a 20, and can use Chromatic Wrath to regain 500 hit points if Tiamat is ever beaten. This combines with Mythic Actions like Hurl Through Avernus which are likely to just banish a character to another realm, all while the Aspect of Tiamat breathes out a variety of its breath attacks that can straightforwardly devastate any defense.
There is a certain trick in learning how to use characters like this that new DMs simply aren't ready for. They are massive top-level threats requiring that players be near a maxed out level themselves, meaning that on top of mastering the monster being run, a DM needs to have a full understanding of their players' full range of spells and abilities that are critical in making their success possible. Dungeons & Dragons is a collaborative storytelling effort. As much as the DM makes up the world, it's the characters who can participate in and shape that world. DMs who throw their players again impossible threats are a bad cliché familiar to far too many players. Getting to run the most epic battle possible with the Aspect of Tiamat is a worthy dream, but getting there is going to take a lot of hard work. DMs and players alike need to be ready for it, and when they are they may just have the most epic session of their lives.
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