One of the new powers that Space Marine Librarians get access to is called Phase Form (WC1 power from the Geokinesis discipline). This power gives Move through Cover to a target friendly unit, plus gives Ignore Cover to its shooting and also allows the unit to shoot at any enemy in range, even if it does not have LOS to that enemy.
Obviously, quite a potent ability, especially compared to Perfect Timing from Divination discipline, since Perfect Timing only affects the Psyker and his unit, while Phase Form can be cast on a friendly unit within 24″ of the Psyker!
There are tons of potential application for this power in an AM army with an allied Space Marine Librarius Conclave, but one that jumps immediately to mind is using it on a squadron of three Leman Russ tanks. Imagine the following:
-LRBT Squadron — 3 x S8 AP3 Pie Plates with Ignore Cover are quite deadly to anything but Terminators and heavy armor, especially given how cheap a baseline Leman Russ is points wise (and the 72″ range Battle Cannon combines VERY well with the ability to ignore LOS).
–LR Exterminator Squadron with Lascannon – 12 x S7 AP4 and 3 x S9 AP shots, all with Ignore Cover, are great at taking out enemy medium armored vehicles, especially skimmers like Wave Serpents and Land Speeders and DE Raiders/Ravagers/Venoms that rely on Jink saves.
–LR Vanquisher Squadron with Lascannon, MM Sponsons — 3 x S8 AP2, 3 x S9 AP2, and 6 x S8 AP1 shots, all with Ignore Cover and no LOS restrictions, are a major threat to vehicles, elite Infantry, and Monstrous Creatures alike.
–LR Demolisher Squadron — It goes without saying that 3 x S10 AP2 Ordnance Large Blasts, all with Ignore Cover, are going to be target priority number one for many enemy armies, since they can threaten pretty everything except flyers and FMCs.
–LR Punisher Squadron with triple Heavy Bolters — 87 x S5 shots with Ignore Cover can be brutal against Light Infantry in cover (like Orks or guardsmen or Cultists) and can also be used against light vehicles or even Bikers with strong Jink saves (like Ravenwing).
–LR Executioner Squadron (with Pask) with Plasma Cannon Sponsons — 15 x S7 AP2 Small Blasts, all with Ignore Cover and Preferred Enemy (from Pask) is the bane of basically anything that is not a vehicle (and can even threaten light vehicles with massed S7 shooting).
Anyway, there are quite a few powerful combinations available to Astra Militarum with allied Space Marine Librarians, but this definitely stands out as one competitive option.