Basic outline of a plan below. I don't have time to put together anything really complicated on the Deep One side of things, but our social stats are absurd and we're killing the Intellect Devourers either way. This just makes their deaths hurt the ones who sent them.
[X] Plan Wages of Blood
-[X] The Sept
--[X] The Red Abishai is to be bottled and taken for full interrogation followed by execution. Exact method to be determined, but most likely via Yss to deny Tiamat the return of her invested power.
--[X] As for the cultists, examine the traces of blood magic upon them with the aim of ensuring that Tiamat may not easily pick a new focus for her power here in Oldtown from among their number. Wild Arcana a
Brain Spider spell to examine their minds for any further information on the cult that might be of import, then if nothing can be found, leave them to their fate at the hands of Oldtown justice. If any have done more than simply be deceived, ensure that this information is passed on to the relevant authorities so that they might be judged for those crimes as well.
--[X] Leave Malarys and the Erinyes to guard the Sept until such time as the Watch or Lantern Bearers can be brought in to take the cultists into custody.
-[X] The Manse
--[X] Lormar and his apprentice are our prisoners. They will be bottled and taken for full interrogation and probably one of either execution or turning.
--[X] Inspect the minds of the Golden Company soldiers with the same casting of Brain Spider (it lasts 1 minute/CL and can inspect the minds of up to 8 people as a standard action) and get everything you can from them before explaining in simple terms the options available to them. Make certain that they understand that any attempts to bring the worship of Tiamat into the Night's Watch shall be met with your...poor humour. They can draw their own conclusions. They can face death or the Wall.
--[X] Make sure to retrieve the 'Darkenbeast' and dispose of it completely.
-[X] Deep One Containment
--[X] Bottle the Greater Intellect Devourer and have it placed within a lead-lined box before securing it within your cloak. It will be used as payment for the Merling King's aid against Tiamat.
--[X] Explain to the Lantern Bearers that there does exists a power utterly opposed to the Deep Ones, a God of Sea who would reward the spilling of tainted blood in his honour in ways that would not be unwholesome. The Merling King can offer many boons from the sea; cleansing the foulness of Deep One power from person and place both - make note of the Bilestone present in the city - and more besides. For abominations like these creatures, turning their deaths against their master would surely be to the aid of all that stand against them.
--[X] If the Lantern Bearers accept this premise, outline the manner in which one would sacrifice these creatures safely to the Merling King, and offer to transform the lesser monsters into harmless animal forms until the time comes. Suggest that an altar (or maybe more, once the Conclave has passed) to the Merling King could bring with it many advantages in Oldtown's war against Deep One machination.
--[X] Finally, be sure to fire off a few Divinations to ensure that the Deep One cell didn't drop anything into the sewers during the battle.
The implications of how the final line could lead to Clerics of the Merling King running around protecting Oldtown when Clerics of the Seven
aren't should be lost on no one