Mordenkainen's is a D&D supplement book organized into chapters based on pairs of related opposites. The idea here is that Modenkainen, who was a wizard character played by Gary Gygax himself, was not a champion of good or evil but rather of balance between the opposing forces of the multiverse. The book is very heavy on flavor text, which I like, and can be used to add depth to and inspire campaigns.
The first chapter deals with Devils and Demons, which are very different things in the world of D&D, and the ongoing Blood War between the lawful and chaotic factions of evil. I love the idea that Hell exists as a barrier protecting the universe against something even worse. Second is a long chapter about Elves and their evil splinter group the Drow. My main takeaway from this was that Elves have a remarkably mellow pantheon of gods who ask very little of their followers and expect to be asked for very little in return. I could get behind a religion like this. Dwarves and their evil splinter group the Duergar come next, not much new here other than the authors seem to have some sympathy for the Duergar, Next comes the Githyanki and Githzerai, the opposed factions of the Gith that travel the multiverse fighting the Mindflayers and each other. My favorite detail about the Githzerai is that they live in a city built on the corpse of a dead god floating in the Astral plane. The last of the flavor chapters is about Halflings and Gnomes. Based on what had come before I was so hoping that a massive lore change was coming in which it would be revealed that Halflings and Gnomes split off from the same race and that an ancient war was fought between the two groups. Alas, it was not to be. It appears that the authors only put the two groups together in one chapter to match the structure of the rest of the book.
The second half of the book is a bestiary with some new monsters and some updates. Shadar-kai are now a subgroup of Elves, which is a change I don't care for. Elves are Fey creatures and all Fey originate in the Feywild. Shadar-kai come from the Shadowfell. I liked them better as their own category of shadowy humanoids. The update to Trolls is interesting, with variants based on environmental mutation. The Sibriex is extremely gross and could be fun to use with its warp creature ability. Several commentators have made fun of the Giff, but I like these hippopotamus based humanoids with their sort of colonial era British military motif. I also enjoyed the look at the second tier dukes of Hell, like Bael, Hutijin, and Titivilus, and their psychological motivations.
TL/DR: Probably the best 5th edition supplement to date.