Second books are tricky. The first book in a series you can pause the plot to do some world-building, and in the third book threads should be wrapped up, but a good second book should not only unwind the story, but test the characters in some way. Thunderhead is decent, but doesn't show any movement in its protagonists.

Citra, now Scythe Anastasia, is the leading light of the old guard, Scythes who believe that taking a life should be a sober business for all concerned. Rowan, now the illegal Scythe Lucifer, is enacting a one man murder spree on Scythes who he believes have fallen to corruption. This business is more fraught than expected, because it turns out that the arch-new wave Scythe, Goddard, surprisingly survived the holocaust at the end of the last book and has a plan to remake human society in his image, with Scythes as the alpha predators at the top.

While the old characters are pacing through the plot, two new characters come in. Greyson is a young man with dreams of serving the AI that rules humanity, and who is dispatched into a secret mission into the Unsavory underground of controlled dissent. And the Thunderhead itself is the last character, each chapter beginning with a short essay on its responsibilities and limits of power. Because while the Thunderhead is near-godlike, that's the difference between one trillion to the one-trillionth power, and actual infinity. Furthermore, strict legal and architectural limits prevent the Thunderhead from interfering or communicating with Scythes, which means that it is significantly hampered in its mission of guiding humanity.

Everything builds towards an appropriately satisfying action-set pieces on a sinking artificial island, and a cliffhanger of an ending that could see Citra and Rowan pop up again in a few days to a few centuries, but this is mostly a book of stasis.