The Blood War

The Blood War is a relatively short and optional side quest that is initiated and completed in the Watcher's Keep Teleport Maze dungeon level.
This quest may not be obtainable based on the party's membership parameters. Both the Tanar'ri and Baatezu encounters have different criteria for whether the party will be rejected at first contact, and then a second check is performed to see if the dialogue can continue or if hostilities will commence. The game provides no clues or warnings ahead of time for this dynamic, so realistically the player couldn't know this.
==Aquisition - Tahazzar==
Requirements: if Gorion's Ward is a Lawful Good cleric or paladin, they will be refused immediately. If the "LastTalkedToBy" character is any Good alignment, or Lawful Neutral, they will be refused in the second check. Party order is important for the second check.
Agreeing to perform a task from the demon Tahazzar will register this side quest in the player's Journal, as follows:
:Journal entry: The Blood War
:I have agreed to do a job for a tanar'ri balor named Tahazzar to find a baatezu pit fiend named Ka'rashur elsewhere in this maze and to destroy it... and then return to Tahazzar with its heart. He has promised me not only a material reward but also the goodwill of the tanar'ri.
==Aquisition - Ka'rashur==
Requirements: if Gorion's Ward is a Lawful Good paladin, they will be refused immediately. If the "LastTalkedToBy" character is Lawful Good or Neutral Good, they will be refused in the second check. Party order is important for the second check.
Deciding to perform a task for the devil Ka'rashur will register this side quest in the player's Journal, as follows:
:Journal entry: The Blood War
:I have agreed to do a job for a baatezu pit fiend named Ka'rashur to find a tanar'ri balor named Tahazzar elsewhere in this maze and to destroy it... and then return to Ka'rashur with its heart. He has promised me not only a material reward but also the goodwill of the baatezu.
==Gameplay==
The party will encounter the Tanar'ri leader Talhazzar or the Baatezu leader Ka'rashur at some point in the exploration of the Teleport Maze.
This side quest can be missed or not materialize for several reasons. First, the party's composition may be incompatible with the requirements/restrictions placed on striking a deal with either of the fiends. Secondly, the party can choose to destroy the fiends and fight them, or the same result can occur through refusing to take on the job. 
If the party is using and following the Madman's Journal to navigate a path through the maze (or perhaps following the Watcher's Keep Teleport Maze Walkthrough), then it is likely that the demon Tahazzar will be the first fiend encountered.
However, if the party is making its own way and forging a unique path, then it is possible that the devil Ka'rashur is the first fiend found.
Once the party has accepted the job and had the quest entry registered in the Journal, then this sets a "global" in the game files that precludes the player from making the same deal with the rival demons/devils. 
In fact, both Tahazzar and Ka'rashur will sense if a deal has been struck with the other group, leading to immediate hostile reaction. For example, if the party has accepted the job from Tahazzar, then when the party finds Ka'rashur, the Pit Fiend will say:
:Tanar'ri! Tanar'ri in the guise of mortals; it is so! The Blood War anew—kill our ancient foe! 
and a similar reaction from Tahazzar will be made if the party has a deal with the devils, saying:
:I smell the Blood War upon these, these mortals! Baatezu conspirators all! Kill them!
To advance the quest, the party will need to defeat the targeted quarry, killing the fiend and obtain their heart as a sign of completing the deed - either Tahazzar's Heart or Ka'rashur's Heart.
This can only be done through force of arms.
Return the grisly proof to the requisite quest giver by backtracking and returning to Tahazzar or Ka'rashur, as appropriate.
The party will have dialogue with the fiend, and it'll ask if they have the heart. Select from the responses offered. 
If the party says no, but they intend to find it and return, then the fiend will wish you well and the quest continues.
If the party says no, and that they have no intention of getting it either, then the fiend will make a remark in disgust, and then become hostile (and the quest is shuttered).
Should the party have the heart, the fiend will have some dialogue and be appreciative, and automatically claim the item from the party's inventory. At this time the party is awarded a magical item - the Thieves' Hood, a Scepter Gem and (quest type).
==Solved quest - Tahazzar==
When the fiend takes Ka'rashur's Heart, it'll say:
:Mmm... most excellent of thee, fleshling. You have earned the gratitude of the tanar'ri. Odd, however, that we still cannot open our gate. Bah! Here is your reward... now begone and leave us be.
At this time the player's Journal updates the quest:
:Journal entry: The Blood War
:I have destroyed a baatezu pit fiend named Ka'rashur and given its heart to a balor, Tahazzar, as requested. Surprisingly, the demon seems to have lived up to its part of the bargain.
==Solved quest - Ka'rashur==
When the fiend takes Tahazzar's Heart, it'll reply: 
:You have the heart of the balor! You have our gratitude forevermore. *hisss* Strange, though, that we still cannot gate... perhaps the tanar'ri had naught to do with our fate?
At this time the player's Journal updates the quest:
:Journal entry: The Blood War
:I have destroyed a tanar'ri balor named Tahazzar and given its heart to a pit fiend, Ka'rashur, as requested. Surprisingly, the pit fiend seems to have lived up to its part of the bargain.
==Gallery==
OG WK TM Tahazzar 3.png|Example screenshot of Tahazzar and the party dialogue and the initiation of The Blood War quest.
OG WK TM Karashur 1.png|Example screenshot of Ka'rashur sensing the bargain with the Tanar'ri, and going hostile immediately.
OG WK TM Bloodwar Quest Complete.png|Tahazzar receives the Baatezu Heart and rewards the party. The quest is finished and the Journal updates.
==Mod gallery==
:Portraits from Portraits Portraits Everywhere
GORTAN1.jpg|Tahazzar - "We tanar'ri have been drawn here to this interminable maze... and we know not why. Try though we might, the planes be closed to us."
GORBAT1.jpg|Ka'rashur - "To this maze we have been brought, led by the force we long have sought. We wish to return to Baator again... but our attempts to gate are all in vain."