Death's End is the thrilling conclusion to the trilogy that began with The Three Body Problem. Da Liu finally lets his imagination soar, and it goes to some truly thrilling places: a new golden age built on deterrence between humans and Trisolarians, a brief and vicious occupation, higher and lower dimensional spaces, fables containing hidden wisdom, and the fate of universe built on the terrifying logic of the dark forest.

Death's End protagonist is Cheng Xin, a young woman and scientist who plays a key role in many events of the book. Cheng is part of mission that sends a probe to the Trisolarians, then the sole person responsible for the red button of nuclear deterrence, and the key vote on the experimental technologies that might make the difference for the human race. Repeatedly, Cheng chooses the softer 'humanitarian' option, and her choice is followed by mega-death, though I am unsure of her ultimate accountability. The universe of Death's End is fundamentally hostile to life, and anything as weak and planetary as humanity. This is a universe of lethal killers, who warp the laws of physics as a weapon.

The key technology of the book is a space-warping drive, which can be used as a lightspeed starship drive, and defensively to wrap an entire solar system in an artificial event horizon, a two way shield by which a species can both protect itself from cosmological hunters, but also prevents any expansion into the galaxy. I'm torn about Cheng as a protagonist, but she's a step up from Luo Ji in terms of characterization, if not up to the very real pain of Ye Wenjie.

It's a cool book, but it's only loosely connected to the people and themes of the prior books in the series. The whole series has pacing issues, with even more cosmological stuff thrown in almost as a coda. Grand space opera is harder than it looks, but I wish Liu had gotten to the firework factory sooner, with space folding and pocket universes and that grand weirdness.