From her vantage point Kaylyn could see the battle play out.  She watched as he companions were doing fairly well against a demon lord.  She recalled the horror of seeing the thoughts of the one demon lord back in the library and she had no desire to fall under the gaze of this one.  She kept her distance and continued to send eldritch energy towards Demogorgon.

Attack: 14 Damage: 13 + 3 to roll Force damage 

Attack: 27 Damage: 8 + 6 to roll Force damage 

Attack: 32 Damage: 19 + 5 to roll Force damage 

The magical barrage continues from the dark cloud concealing Kaylyn, but no matter how many hits Demogorgon takes, he just seems to become more feral and ferocious. Kirin draws closer, although with his eyes averted his second projectile flies wide. The taunts from Pax turned his heads, but the mad dash from Rhayna brings her within his grasp, and like an enormous demonic hunting cat, he pounces instead on Rhayna, pounding at her with his deadly tentacles...

Tentacles v Rhayna:
Attack: 27 Damage: 44
Attack: 32 Damage: 34
For each attack that hits, Rhayna's HP Max is reduced by that amount unless she passes a DC 23 CON save (for each)

I forgot the Couatl again, so Rhayna can give an order there, then we will continue with Sekkar.

MAP:

(Wait, Couatl had the order to cast Greater Restoration on Sekkar, did it work?)

Seeing that Sekkar needed desperate help, she tried to make her way towards the Lizardman, but she didn't realize that it put her in danger as well.

CON Save (DC 23): 18

CON Save Concentration: 12

CON Save (DC 23): 8

CON Save Concentration: 26

(At the very least the Couatl is still hanging around!!)

(Holy Moly. Rhayna doesn't look very spry at the moment. She's down to 1HP!!!!)

(I forgot about the Couatl as well.)

The Celestial creature touches Sekkar and heals 8 HP.

Yes, the greater restoration worked on Sekkar. His current hit points didn't change, but his hit point maximum is back to its full amount.

Just some housekeeping... to make sure we are seeing the same numbers...
- Rhayna took 85 damage (45 from the first tentacle attack and 40 from the second)
- The CON saves failed, so her HP Max is also reduced by 85
- She was on full health (93/93) before the attack, so should be now on 8/8. Currently the character sheet shows her in death saves.
- The DC for the concentration checks are 22 and 20 respectively. The first roll posted is only 12.
- If this is correct, the Couatl still gets to cure wounds on Sekkar before it is un-summoned (on Demogorgon's turn)

(Rhayna started the combat with 12 temp HP.)

In fact you all receive 25 temp hp at the start of each round anyway, which overrides any you started with. As it turns out, they don't help that much if you are hit by Demogorgon's tentacles and fail the CON save, as the damage also reduces your HPMax by the same amount. So if you have 93+25 out of 93 hp, and you take 85 damage, you first lose your 25 temp hp, then lose another 60 of your own hp, which would reduce you to 33/93. However, if you fail the CON saves, the HPMax is reduced by 85, from 93 to 8. And since you can't have 33/8 hp, you in fact go down to 8/8.

I got the idea of temp hp per round from the final battle of another campaign I was a player in (as 'Badger the gnome wizard'). I thought it would be a good idea to include it for this final battle, but didn't really factor in the effect of the HPMax reduction. Still it isn't a total waste.... the tail attack doesn't reduce your HPMax.

Speaking of the tail attack... well... we'll get to that at the end of Sekkar's turn.

Ah! That felt really good. Surprisingly so. Sekkar still felt every bite from those tentacles, even the bleeding had barely stopped, but somehow the colours returned to the world and he did not feel like the last gasp for the air.

Not so much for Rhaina, though. It was impossible for him to stand toe to toe with the Demonlord, he doubted, any amount of holiness would save the girl either. He assessed the distance... What he was about to do was illogical, stupid and counterproductive to the mission, but he can not just let the monster annihilate one of his tribe!

Movement (including Dash from BA): all the way back to Demon

Attack: 5 + 25 Damage: 16

Attack: 10 + 14 Damage: 10

SA to attack that hits: 14

(Thanks for the clarification about Temp HP and HP Max reduction. The math was a little tricky.)

I'm gonna be an a-hole and say... Sekkar can't make it that far. He is starting 65 feet from Demogorgon, and even with a Dash he is still out of reach for Dawnbringer. And with a shield on, it would take more than a free action to draw another weapon. Unless I've missed something, in which case please correct me.

Don't be mad at me. Be mad at Demogorgon. He's the real a-hole here.

Diagonally he can, but as you say. How about dropping the shield, using BA for Steady Aim and shooting? Would it work?

Dropping a shield takes an action

(Sorry, Rhayna, I tried!)

If holding Dawnbringer with the jaws allowed, Sekkar will mover 30ft and throw Handaxe (Attack: 8 + 17 Damage: 12 ) to attract Demons attention, and summons Healing Spirit on himself for 6.

If it's not the option - only movement and the spirit.

Holding the sword in the jaws is super-cool, therefore totally allowed. :)  

In her effort to race to Sekkar's aid, the impulsive Rhayna inadvertently puts herself in the demon's reach, and she takes her turn at feeling the devastating drain on her life-force that accompanies the biggest single pounding of her life. Sekkar himself in turn runs to Rhayna's aid, and although his accurately-thrown axe strikes the demon, is seems to have no effect other than to make the fiend aware of his presence. Demogorgon turns, his heads swiveling as they survey the battlefield, assessing each threat - the almost-dead lizard warrior, the two mages, another entity still beyond his vision... and the human wielding divine magic, that conjured the hated celestial and trapped him momentarily in black goo. 

Slowly, with a deep snarl, Demogorgon turn to face Sekkar. He flexes his tentacled arms, and one head bellows a roar that is unmistakably a challenge to battle, as a deep voice echoes in Sekkar's mind: I SHALL DEAL WITH YOU NEXT... AFTER THIS ONE! Then his spined tail swings behind him.. at Rhayna.

Demogorgon Tail v Rhayna
Attack: 30 Damage: 49 bludgeoning plus 27 necrotic

(...and unless anyone can prevent it...)
Rhayna's companions look on in horror as her body is crushed under the powerful tail, leaving her motionless and lifeless on the ground.

...

Pax is next.

 MAP

Pax watches in horror as Rhayna seemingly runs directly towards the Demogorgon.....like some sort of sacrifice...rushing all alone right at it. Too far away to help, Pax can only watch in horror as the mercurial girl is grabbed by the powerful demon and the life is smashed out of her. A moment passes...then another....as the impact of her dead friend settles in like an icy dagger to her soul.  And then Pax draws breath. Resolve....hard like a diamond forms within Pax's soul.....forming from a internal whirlwind of anger, sadness and hate.  But even now haste was not what was needed.  She needed to draw this out. She saw the blasts of magic from Kaylyn, saw the demon's wounds. It must fall eventually...she just needed to buy time.  And hurt it some more.

She lets loose a blast of lightning from the swirling storm at the creature, and then three blasts of eldritch energy and screams at the thing with all her rage, "DO YOU FEEL YOUR DEATH COMING FOR YOU. AT MY HANDS?!  DO YOU QUIVER IN FEAR!?"

Lightning blast : Attack: 23 Damage: 22 lightning damage.

Eldritch blast: Attack: 21 Damage: 12

Eldritch blast: Attack: 28 Damage: 11

Eldritch blast: Attack: 25 Damage: 10

As she blasts at it, she strides forward slowly. (15 feet "up") 

Missed my bonususussss....

Whether Pax's screams are simply ignored by Demogorgon, or whether they are lost in the intervening vortex of wind, there is no visible response from the demon lord. Even as the lightning and magical blasts continue to strike the beast, it seems not to notice the damage, although Pax can see black ichor oozing from multiple wounds. Perhaps the fiend is too caught up in a fervor of killing to notice pain - if it even feels pain...

As Demogorgon prepares to finish off Sekkar, one of the heads turns and glares at Kirin, the demon's gaze threatening to reduce the wizard to an inert blob of fear and confusion...

Kirin: Please make a DC 23 WIS save or become stunned.
You succeed automatically if you avert your eyes, but if so you cannot see Demogorgon until the start of his turn.

Then it is Kaylyn and Kirin to go next.

(Map as before, but with Pax an extra 15' north)

(Sigh gambling time)

Wis Save: 21 + 4 = 24 (just makes it)

Kirin takes a moment to take the full sight of the demon lord near him as he points his staff at the demon as his other hand probes around for his dagger, only melee weapon on him, sending a another star hurling towards the great Beast.

Attack: 21 Damage: 14

Bonus Roll to hit: 9

He takes a moment to breath deep as he mentally prepares for the backlash before making a throw motion with the dagger while casting Steel Wind Strike over channeling it, arcane energy explodes front of him as the very air lash out at Demogorgon and dark energy rips at Kirin. 

Attack: 23 + 5

Damage: 60 Force Damage .... make that 120 Force Damage

Backlash to Kirin:28 + 11

Kaylyn watched in horror as Rhayna fell.  She knew she couldn't stay so far out anymore and needed to get closer to help. She would have to risk getting closer to the the demon lord

She then teleported by opening a Dimension Door to stand next to Rhayna.

She realized Rhayna was dead and so turned her natural ability to heal using her healing light and Sekkar glowed in a white light for a moment and is healed for 13

I think Sekkar would be within 60 feet