I'm all for endgame content being available on all map tiles, but them all being pits of demons...... no, I don't think it's a good idea. Pits of demons (actually more like passageways to demon cities) sealed off by adamantine "plugs" in times before time sound about right for mountains, but in plains, swamps, and other places there should be a lot more different stuff.
Under plains, spacey halls of hostile undead creatures could be found. Under evil plains, halls of undead creatures and a single Ultra-everything Lich Lord. Good plains may be guarded by ethereal warriors or martyrs, that are hostile nonetheless for breaching their grounds. This is all "in memory" of battles waged on open plains. Graves or portals to the other side could be used to prevent lag until the site is discovered. The loot is not adamantine, but a healthy complement of high-quality equipment and riches, including artifacts.
Swamps may be inhabited by more slithery and poisonous creatures. Snakes, spiders, toads, and various variations and crossbreeds thereof. Large breeding grounds and hives of these evil creatures could be found under swamps, led by especially large specimens. The loot in this case is webbing, and skins and scales of very rare specimens of animals, plus rare gems that could have been gathered by the hive.
Forests could be covering ancient elven ruins, which have entryways to a dimension of exceptionally powerful beasts, like dragons. Black dragons, blue dragons, etc.
So yes, I'm all for endgame stuff under everywhere, but only demons is too predictable, even if hardly containable.