Need help with ending my buppido encounter im running tomorrow. It's also my sons 14th birthday so I've really tried to make this memorable.

I know this is long but it's my personal written out guide to the custom encounter. I need to know 3 things specifically. But I'm always open to other ideas and input.

1) does the encounter sound/ feel epic?

2) does the way buppido dies feel good?

3) how do I finish it? I'm stuck on what to do next?

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The room layout

As you enter you see 10 cultists forming a semi circle around a raised platform kneeling and bowing while chanting rhythmically. On the platform you see an alter behind which you see the 5 npcs and Asha are chained to posts surrounding what looks like a summoning circle. 5 drow (from velk) are chained and kneeling on the first level of the platform infront of the alter surrounded by 3 cultists guards. 1 drow is being held down over an alter by 2 derro cultists. Behind the alter stands buppido with a large rod in one hand and a knife in the other. In the corner is a cage containing Stool and the 5 hook horror babies.

Buppido sees the PCs enter and welcomes them.

     "Your finally here, id begun to wonder if you'd received my message. I've been waiting along time for this,  everything Is exactly as I planned it and you were all to foolish to see the real strings behind the act"

He looks down to the drow being held down infront of him, raises his free hand with a slithering blade, utters something softly, and then he brings the blade down and slits the throat of the drow., Immediately the 2 cultists holding him down pick him up and hold him suspended so the blood runs down the alter and into the summoning circle. He looks back up to the PCs and ushers

        "Come watch as I shed this mortal prison take my place among the gods as  Diinkarazan brother of Orcus and Master of the undead who will return me to my proper form."

He then looks to the 2 guards and nodes his head. They chop the drows other limbs off and toss them to the side of the platform and it tumbles down.

If pcs approach to stop buppido he sends the 10 cultists that are bowing at them. As he continues to torture and kill drow npcs ritualisticly.

...............Fight 1 occurs (10 Derro)...................

During the fight he kills a couple Drow npcs. The 5 remaining cultists continue chanting in unison.

Once the 10 are killed buppido laughs.

       "This is all apart of my divine plan. You being here now only proves my divine ascendancy is at hand."

He holds up his hands strait out, then raises them. As he does this the bodies of the recently killed cultists twitch and twist with a sickening crack of bones and all 10 Derro raise as zombies.

.............Fight 2 happens (10 zombies).................

Buppido laughs,

   ""YES, YES everything is going to my divine plan, there is no stopping what's already fated to pass, you are all powerless against me"

The chanting raises, you see the final drow squirm as his throat is slit and you can audibly hear the name "ORCUS" ring out

Buppido then holds up his hands and swirls the rod in the air, as this happens the blood in the summoning circle starts to swirl and forms up into a twister. The 5 remaining cultists scream and wretch as you watch ghouls claw thier way out from thier mouths splitting open thier bodies into pieces left on the floor of the platform.

                 Madness throw#1

.................Fight 3 occurs (5 ghouls and 1 derro servant buppido)..............

When the killing blow is felled on buppido he immediately uses a lair action and unleashes a blast of energy out that pushes all pcs back 30 ft and paralyzes them (DC 20) he then dashes back towards the summoning circle (if not there) and crys out

"Orcus.... come, return me to my proper form as a God"

He releases his chromatic blast into the summoning circle. A treacherous crack is heard and the blood twister splits out and into the mouths of each of the npcs who are chained up as a horizontal portal is opened. The chained up npcs life force is clearly seen to be fading.

                Madness throw #2

Thunderous sound is heard and the ground begins to shake as the murderous form of Orcus begins to take shape out of the blood portal. Buppidos chromatic beam still blasting into the blood. Buppido turns to the pcs

          " you have fought valiantly. But you are no match for my power,  for the power of the abyss"

More definition of limbs and of the wings of orcus can be seen as a blood curdling ROAR erupts from the bloody form of Orcus..

                 Madness throw #3

      "I am A God like my brother. Let today be the day we take this corrupt city and deliver it into ruin. We will turn every Duergar, Derro, stone giant and slave into the undead,  our army will be unmatched and will wash over the underdark in a flood of blood. We are unstoppp, AHHHHH"

buppido screams in pain and as he falls down and sees a baby hook horror biting his leg.

       "Why you little shit" 

he pulls back his arm to swing the rod but it's caught by another of the babies mouths as it crunches down and he drops the rod..... A flurry of clicking happens and suddenly all 5 hook horror babies are jumping on, and tearing open his flesh and biting buppido. Behind them is Stool the lingering dust from his spores visible in the air and his eyes clearly focused on buppido. You've seen this look enough to know that he is clearly communicating with the babies. Buppidos screams

           "You have not beaten me, I AM INEVITABLE, I am un kill able,.."

           "this is all apart of the define plan" he mutters 

            "You'll see..... I'll be back... and I will find you all"

His chromatic beam blinks a few times and stops as the babies begin to gnaw into his face.

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I need to know where to go from here.

Do I just have the portal blow up and everyone be OK.

Do I do something to make them have to counter the portal and spell.

I'm just not sure where to go with it now.

I know it's long but thank you to everyone who gets through it.