This could use some additional micromanagement, but I think it's a suitable way to use the encounter considering the setting.
I "Almost" included a series of falling ceiling traps in the room to constrict movement and possibly catch an unwary PC in a trap, but I honestly think if I used that the way I wanted to... it wouldn't be reasonable.
Room #29: Infernal Sanctuary
This room is large enough to hold a small banquet, but only one creature occupies these walls. Blasphemy, a Marilith, sees an invasion as nothing less than entertainment. The hallway leading up to her sanctuary is undoubtedly trapped, or perhaps she is simply too confident that such concerns are only meant for lesser creatures.
This room is not lit, because there is absolutely no need for such. Instead, a single unholy alter, 10 feet long, rests at one end of the room. On the opposite side, a simple decorative door separates the hallway from the sanctuary. The sanctuary is a heated place, heated by a vein of magma that flows directly under the stone floor of this chamber. This causes the room to be unbearably hot, full of the rank sweat of demons and victims that come and go. This heat causes 1 point of fire damage every round that a creature stands within the chamber.
Blasphemy has no reason for much more decor, besides her ever comforting bed, a large pewter piece shaped like a twisted mass of vines lays about the center of the room in a shape that could only be comfortable for such a demon. She "rests" many hours of her passing time, waiting for inspiration or excitement to bring out her unholy bloodlust.
When she is alerted to an attack, she would likely organize her own assault within the dungeon, but when trapped in her own room she would quickly call for aid from one of her servitors. She may be confident, but within her own abode the opposition would need to have conquered much of her defenses, and such a challenge should be fought with all that is available. Her home, however, is not a place she would flee from unless absolutely forced.
Combat
Though mariliths thrive on grand strategy and army-level tactics, they love physical combat and never pass up an opportunity to fight. Each of a marilith’s six arms can wield a weapon, and the creature gets an additional three weapon attacks with its primary arm. Mariliths seldom rush headlong into battle, however, preferring to hang back and size up the situation first. They always seek to gain the best possible advantage from the local terrain, obstacles, and any vulnerability or weakness in their opponents.
A marilith’s natural weapons, as well as any weapons it wields, are treated as chaotic-aligned and evil-aligned for the purpose of overcoming damage reduction.
If Blasphemy has knowledge of an attack, her immediate action is spent using her Summon Demon ability. Dretches are for large parties of significantly weaker enemies, so Blasphemy will summon 1d4 Hezrou unless she has already determined that the hezrou's Blasphemy magic may be too weak to affect her opposition (hezrou's Blasphemy does not effect targets of 13th level or higher). If this is the case, she will choose between a 50% chance of summoning a nalfeshnee and a 20% chance of summoning an additional marilith to her side. The nalfeshnee is both preferable because of the improved chance of being summoned, and a nalfeshnee's ability to deal with spellcasters more efficiently than any marilith.
As her foes near, she will quickly prepare in coordination with her summoned minions, assume that a second Marilith would act in the same fashion as Blasphemy:
1st round - Cast project image (DC 23) to Blasphemy's side; Nalfeshnee readies and action to cast greater dispel magic on the area past the doorway when it is opened/destroyed OR hezrou move to corners on the door-side of the room and ready actions to cast Blasphemy when 2 or more targets enter the room.
2nd round - Cast blade barrier (ringed wall 5 foot radius centered 5 feet away from the main entrance) if aided by a nalfeshnee, otherwise ready an action to cast blade barrier once the doorway is opened, and move so that there is no line of sight between the doorway and Blasphemy till a target moves into the door.
3rd round - Cast magic weapon on 3rd longsword, Blasphemy takes a move action to direct the projected image to the center of the blade barrier, or continues to hold readied action
4th round - Cast magic weapon on 4th longsword, or continues to hold readied action
5th round - Cast magic weapon on 5th longsword, or continues to hold readied action
6th round - Cast magic weapon on 6th longsword, or continues to hold readied action
Hezrou Scenario
The hezrou will attempt to cast multiple blasphemy spells, but these spells will not have an effect on the party if they are level 14 or higher. If the party is in fact vulnerable to this series of spells, they will continue to disable the invaders with blasphemy spells while the marilith engages the enemy in melee. Given this choice, the readied blade barrier should be cast behind the invading party in the hallway leading up into the chamber once the chamber is infiltrated, effectively preventing them from retreating from the horrible melee ferocity of a fully prepared marilith. Otherwise, the hezrou may be forced to engage in combat as best they can.
Nalfeshnee Scenario
Once combat begins (keyed by the doorway to Blasphemy's chamber being opened/removed in some manner, and if the nalfeshnee is present, a sudden dispel magic being cast against the invaders to remove buffing spells), the blade barrier acts as a literal wall (15d6 force effect, save DC 23 for half).
The nalfeshnee acts as a spotter and dispelling aid to the marilith while she fights. The projected image, until disbelieved, is directed by Blasphemy to draw the enemy into the barrier (which would of course be extremely bad). After this tactic is disabled or rendered useless, the Nalfeshnee will cast greater dispel magic on the area which contains the blade barrier and the projected image, and Blasphemy will move 5 feet (into the space previously occupied by the blade barrier) and use her 10 foot reach to perform a full attack action against any target in range, unless the nalfeshnee communicates that the attackers are further down the hall, in which case Blasphemy will wait for them to enter the room willingly, which will give her the opportunity to make use of her reach and massive number of attacks.
After the battle reaches this point, it should turn into the regular chaos of combat, with the nalfeshnee casting dispel magic and feeblemind on every target that presents itself, unless Blasphemy is losing the fight. If Blasphemy appears to be losing, the nalfeshnee can let loose it's smite ability to stagger the enemies offense long enough for the Marilith to continue the assault unimpeded.
Second Marilith/No Support Scenario
With a second Marilith, Blasphemy will be without fault by simply engaging the enemy directly in melee combat, and surrounding the "combat ring" with a blade barrier to cripple her enemies’ ability to retreat. If Blasphemy is caught without aid, she will likely be reduced to the same tactic of using her spell like ability to cast blade barrier in conjunction with her might in melee. When alone, she is most likely to use her greater teleport ability to escape the battle, or harass the invaders.
Blasphemy
Large Outsider (Chaotic, Extraplanar, Evil)
Hit Dice: 16d8+144 (216 hp)
Initiative: +4
Speed: 40 ft. (8 squares)
Armor Class: 29 (-1 size, +4 Dex, +16 natural), touch 13, flat-footed 25
Base Attack/Grapple: +16/+29
Attack: Longsword +28 melee (2d6+12/19-20) or slam +24 melee (1d8+9) or tail slap +24 melee (4d6+9)
Full Attack: Primary longsword +28/+23/+18/+13 melee (2d6+12/19-20), and secondary longsword +27 melee (2d6+6) and 4 longswords +25 melee (2d6+4/19-20) and tail slap +22 melee (4d6+4); or 6 slams +24 melee (1d8+9) and tail slap +22 melee (4d6+4) Space/Reach: 10 ft./10 ft.
Special Attacks: Constrict 4d6+13, improved grab, spell-like abilities, summon demon
Special Qualities: Damage reduction 10/good and cold iron, darkvision 60 ft., immunity to electricity and poison, resistance to acid 10, cold 10, and fire 10, spell resistance 25, telepathy 100 ft.
Saves: Fort +19, Ref +14, Will +14
Abilities: Str 29, Dex 19, Con 29, Int 18, Wis 18, Cha 24
Skills: Bluff +26, Concentration +28, Diplomacy +30, Disguise +7 (+9 acting), Hide +19, Intimidate +28, Listen +31, Move Silently +23, Search 23, Sense Motive +23, Spellcraft +23 (+25 scrolls), Spot +31, Survival +4 (+6 following tracks), Use Magic Device +26 (+28 scrolls)
Feats: Combat Expertise, Combat Reflexes, Multiattack, Multiweapon Fighting, Power Attack, Weapon Focus (longsword)
Challenge Rating: 17
Treasure: 8000 gold coins; 35 gems; +3 large longsword and +2 large longsword
Alignment: Chaotic evil
Constrict (Ex): A Marilith deals 4d6+13 points of damage with a successful grapple check. The constricted creature must succeed on a DC 27 Fortitude save or lose consciousness for as long as it remains in the coils and for 2d4 rounds thereafter. The save DC is Strength-based.
Improved Grab (Ex): To use this ability, a marilith must hit with its tail slap attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it succeeds on the grapple check, it can constrict.
Spell-Like Abilities: At will—align weapon, blade barrier (DC 23), magic weapon, project image (DC 23), polymorph, see invisibility, telekinesis (DC 22), greater teleport (self plus 50 pounds of objects only), unholy aura (DC 25). Caster level 16th. The save DCs are Charisma-based.
Summon Demon (Sp): Once per day a marilith can attempt to summon 4d10 dretches, 1d4 hezrou, or one nalfeshnee with a 50% chance of success, or one glabrezu or another marilith with a 20% chance of success. This ability is the equivalent of a 5th-level spell.
True Seeing (Su): Mariliths continuously use this ability, as the spell (caster level 16th).
Skills: Mariliths have a +8 racial bonus on Listen and Spot checks.
Feats: In combination with its natural abilities, a marilith’s Multiweapon Fighting feat allows it to attack with all its arms at no penalty.