I really dug this one, much better than Draco. A better story, more weirdness, and it lost the dreamlike flair at the end of Draco where he had to rush the entry to the Emperor's throne room.
It picks up 100 years later, with the reawakening of Jaq, Meh'linde, and their Navigator try to discover what happened in the intervening century. To do this, they need an astropath. They find a planet under attack by Slaaneshi cults, their Navigator is turned and then killed, and they meet up with their Squat friend who has been in service to the Harlequin-man/inquisitor. They pick up a Navigator that was secreted away by Grimm, and then seek an audience with the planetary governor. In order to obtain the astropath necessary to eavesdrop on the Warp, they end up discussing Chaos and then executing the governor, his guards, and his harem, knowing that this will doom the planet to fall due to the collapse in morale. Plenty of Dune-style weirdness here.
The navigator and astropath become close. The astropath is able to gaze into the uncovered Navigator eye due to the blindness from soul-binding, and they spend hours locked together. Grimm is also not quite trusted, as Jaq is suspicious of his fortuitous arrival. Jaq and Meh'lindi torture Grimm, and there's a major lore dump here. Apparently the Emperor had hundreds of children that are psychic blanks to him, and are immortal. These are the Sensei. There is a secret order of the Inquisition made up of those that had been possessed by a daemon and then thrown off the shackles of possession that is seeking to gather them. The goal is to sacrifice them to create the Numen, a rebirth of the Emperor akin to Dune's Golden Path.
They also find the former master of the Callidus shrine who ordered Meh'lindi's transformation into a genestealer-only shapeshifter, and convince him to reverse the surgery. Bit of a story hook here; he's in a Dreadnought body, but they leave uneventfully. This is only really important because it lets Meh'lindi mimic Eldar with her shapeshifting.
In eavesdropping on the Warp, they discover that the Eldar are enacting a ritual around the planet that underwent Exterminatus in the first book. A mind-wiped member of the Hydra cabal, Baal Firenze, is to lead a group of Imperial Fist space marines (with our old friend Lex, from Watson's other novel, now a captain). The slaughter is to whatever enigmatic designs the Eldar have, both of Marines and themselves. Meh'lindi assumes the guise of an Eldar Guardian and pretends to take the group prisoner. Jaq et al end up convincing Lex that Baal Firenze is not trustworthy, and he flees into the Webway with them.
They meet a Harlequin band, who give a contrasting view of the Numen - they don't want to sacrifice them, but to control them and replace the Emperor with them. They believe that a resurrected Emperor could be the Dark King, a 5th Chaos god. This is to take place at the Rhana Dandra, the Eldar end times. They are to be executed, but are saved by a timely arrival of space marines who also entered the webway.
Jaq's astropath was able to witness the path to the Black Library, where Eldar keep the most heavily guarded secrets, by staring into the eye of the navigator. The astropath is killed, and Lex carves the rune giving the path into the navigator's eye, shielded by his helmet. The group proceeds through the webway, following the path given by the rune. The Phoenix Lord of the Howling Banshees pursues them, killing the Marines one by one until only Lex is left. She disables his armor, and then kills Meh'lindi. Jaq is distraught, and I was honestly surprised by this - it's rare to kill a principal character, especially one who acts as a sounding board for the main character. Interested to see how this shakes out in the next book.
They enter the Black Library, find the book of the Rhana Dandra, which is studded with priceless gems. Due to the rune they're phase shifted away from the Eldar who would stop them, and only view them as shadows. The plan is to sell the gems to finance the research necessary, since none of the group read or speak Eldar. Jaq adds to his body count by executing the greedy navigator who had been corrupted by dreams of the immense wealth, adding to the trail of bodies. Really like this aspect of Watson's stuff - even the "good" guys leave a trail of innocents behind them in the hope of saving humanity. There's a grittiness to this that I think is left out of 40k stuff where the people doing the murdering tend to be side characters, or justify in some way. Now it's up to Jaq, Grimm, and the now-armorless former Space Marine Lex to try to stop the hydra . . .