The tricks D&D designers used to keep 20th-level characters from breaking the game

polygon.com:

One of the biggest draws of Vecna: Eve of Ruin, the final campaign in the decade-long run of 5th edition Dungeons & Dragons, is the fact that you can play as a high-level character. Most D&D campaigns end around level 10. Eve of Ruin starts there, and then proceeds to go up to level 20, the game’s cap. That means players will begin the story with extremely powerful, potentially game-breaking spells and abilities. So how did the designers at Wizards of the Coast manage to keep characters in line, preventing them from blowing up the entire narrative from the jump? The answer is very, very carefully.

First of all, the danger here is real. High-level magic users can take lots of powerful spells before or at level 10. These could be used to quickly reveal secrets that would spoil things like the true intentions of major characters, or the locations of people or objects. Casual use of those abilities, especially early on, could spoil… well, everything really. So designers have explicitly noted that some areas in Vecna: Eve of Ruin are different from others. That includes the blanket use of Nondetection in particular locations. It’s a spell in the Player’s Handbook that ensures people and objects “can’t be targeted by any divination magic or perceived” by other magical means. Other nerfs aren’t so subtle.

Teleportation abilities were a particularly big concern, it seems. Vecna: Eve of Ruin does a lot of narrative hand-waving to tone them down or disrupt them completely. For instance the hub location for the campaign is Sigil, the City of Doors in the Outlands. It’s an entire plane of the multiverse that can only be entered or exited via a series of obscure portals. Once players leave the hub, there’s no way to get back until the job is done.

In other locations, it’s simply noted in the “Regional Effects” section of a given chapter that certain spells don’t work as intended — including teleportation and summoning spells, and spells that would otherwise allow revealing the