Hmm some concerns about this:
As Bobss said, the genesis of the Maesorus Temple doesn't bother me. It sounds like the kind of thing the oft-hypocritical Imperium would do. Making deals with heretics isn't new and the Assassinorum isn't squeaky clean, so it's not out of character for them to do something questionable like this. The only part of it I am leery of is the deal with the World Eaters, because I don't see the World Eaters as being the type to make deals with anybody, better if he had chosen the Night Lords or the Red Corsairs; a more mercenary group of CSM. The idea behind the Maesorus Assassins is an interesting one, a bit too much like Resident Evil for my liking, but it's a good one and it seems like this Leigenstrasse is an interesting opponent.
The fighting however is the problem. But not all of it. Lysander, a squad of Assault Marines, a squad of 1st Company veterans, a Scout Sniper Squad, the Emperor's Champion, the Culexes Grand Master and the Vindicare Grand Master, and she beat all of them. Too much. But it's the other Assassins that make it too much in my opinion. I wouldn't have a problem with it if it was just the Space Marines she beat, because i've always seen the Assassins as powerful opponents who could do such things. But the idea that this one warrior defeated three squads of the Imperial Fists, one of the strongest Astartes Captains alive, and two Assassin Grand Masters simultaenously is ludicrous. Even a Daemon Prince would have had a tricky time with that, not saying that a Prince wouldn't win, just that it would be hard. But this is a flesh and blood opponent that ultimately is mortal.
And a second fight with Lysander, a Terminator Squad, a 1st Company Epistolary, the Emperor's Champion, the Vindicare Grand Master and an Eversor Assassin and only Lysander survives. Again too much. Take away the Assassins, or in fact just the Vindicaer, and it's not so bad. The Eversor I can accept as it would be a hell of a fight, but the Grand Master is just too much. In my opinion this is a case of trying to show how powerful this Leigenstrasse is by having her defeat opponents that we will automatically recognize as being mighty, but taking it too far by having them be opponents that shouldn't be beaten that easily. If these Assassins had been regular members of their temples I could buy that they'd be beaten, but the Grand Masters?? No, I would think that they'd be able to win with the kind of Astartes backup they had.
Edit: Have just corresponded with somebody whose actually read the book and he had this to say.
Makes the whole situation of the book and the antagonist much better once you understand the context. The fights do sound ludicrous on the Lexicanum but according to David Earle on the bolthole the actual fights are quite fair, and because the Imperial forces seem to get in each other's way which we know is something that happens frequently when Imperial branches work together, especially a pair as different from each other as the Astartes and the Assassins.
Really want to read this book now, so I can get a real look at the events within for myself.