The Mantis Warriors are a White Scars successor, and trade a preference for bikes for excellence in guerilla warfare in dense terrain. As Shadow Killers, all Mantis Warriors infantry have Move through Cover and Hammer of Wrath. Beyond that, they gain Furious Charge when charging from cover. This all sounds pretty good, but the Mantis Warriors lack a means to get into this cover in the first place. The ability to Scout or Infiltrate would be huge for them. As it is, I feel like this rule really benefits assault Scouts, but everything else will gain only situational benefits. In all of that, I think army-wide Move through Cover is the biggest advantage.
The Mantis Warriors’ other special rule, Children of Prophecy, is the reason I think most people will take these Chapter Tactics. This allows them to re-roll failed attempts to Seize the Initiative and allows Mantis Warrior Librarians to use Divination powers. Divination is currently the hottest discipline on the table and Space Marines don’t have direct access to it. This means that Mantis Warriors have a decent advantage over other Chapters if you want to use a psyker heavy army.
Ahazra Redth is the Chief Librarian of the Mantis Warriors, and led his Chapter through the Badab War following the death of their Chapter Master. He has a standard Librarian statline with an extra wound, and has very standard equipment. He’s only a Mastery Level 2 Psyker, so he’s not as powerful as Tigurius or Sevrin Loth. However, his Talisman of Sundered Souls gives him a 5+ invulnerable save and allows him to re-roll his first failed psychic test every turn.
Like all Mantis Warrior Librarians, Redth has access to Divination, though he loses access to Telekinesis for some reason. However, one of his powers is always Mirage, which grants his unit and any vehicle they are riding in Shrouded. Any models charging them count as making a Disordered Charge. All in all, a solid defensive Blessing. Finally, Ahazra Redth’s Warlord Trait grants any unit that he joins Interceptor and Night Vision.
Overall, Ahazra is an excellent support psyker. He’s probably strongest when joined to a backfield firepower unit, as he’ll give them Shrouded, Night Vision, Intercept and he can Prescience them as well.