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Also I’m going to start using the lair actions in Fizbans if thats alright, including the universal ones.

Quote from TheFriendlyArchfey >>

Also I’m going to start using the lair actions in Fizbans if thats alright, including the universal ones.

 I've got some access to that via Content Sharing in a campaign I'm in. But I'm not 100% sure which those are. So I'll be relying on you for the details.

For Ini15a, gonna move side 40ft, forward 40ft, and up 80ft [dash fly]

Lair Action
Ini17 (20,0,0), 2 4th expended, 1HP, polymorphed to Cloud Giant 195HP - 10/2
Ini15a (100,40,80) 1 4th expended, 4 sorcery points expended, polymorphing Ini15a and Ini11, Young Blue Dragon 
Ini11 (140,0,0) away 9HP, polymorphed as Young Blue Dragon
You (0,0,200) (2/3 legendary resistances) (282 HP)
Ini2 (240,0,0) (frightened tbd)

Legendary before Ini11?

Nope.

Ini11 swoops the other direction but same general flight plan

Lair Action
Ini17 (20,0,0), 2 4th expended, 1HP, polymorphed to Cloud Giant 190HP
Ini15a (100,40,80) 1 4th expended, 4 sorcery points expended, polymorphing Ini15a and Ini11, Young Blue Dragon 
Ini11 (100,-40,80) away 9HP, polymorphed as Young Blue Dragon
You (0,0,200) (2/3 legendary resistances) (282 HP)
Ini2 (240,0,0) (frightened tbd)

your turn

Breath weapon recharge: 6

Fly 20 feet up, and then fly until i’m directly above ini15a

That's a dash right?

to (100,40,220)

Ini2

WIS Save

20 or 19

(300,0,0) [that frightened is really making him run)

Yeah its a dash also sorry I meant 40 feet up.

Okay, Lair Action time and

Lair Action
Ini17 (20,0,0), 2 4th expended, 1HP, polymorphed to Cloud Giant 190HP
Ini15a (100,40,80) 1 4th expended, 4 sorcery points expended, polymorphing Ini15a and Ini11, Young Blue Dragon 
Ini11 (100,-40,80) away 9HP, polymorphed as Young Blue Dragon
You (100,40,240) (2/3 legendary resistances) (282 HP)
Ini2 (300,0,0) (finally no longer frightened)

Lair action: lair rejuvenation. I regain 21 hitpoints.

*you regain hitpoints equal to hit dice*

Alrighty.

Ini17 will dash towards the fight

Lair Action
Ini17 (100,0,0), 2 4th expended, 1HP, polymorphed to Cloud Giant 190HP
Ini15a (100,40,80) 1 4th expended, 4 sorcery points expended, polymorphing Ini15a and Ini11, Young Blue Dragon 
Ini11 (100,-40,80) away 9HP, polymorphed as Young Blue Dragon
You (100,40,240) (2/3 legendary resistances) (303 HP)
Ini2 (300,0,0)

legendary action before Ini15a?

Nope.

Ini15a will go 40ft towards the ground and then 40ft towards Ini11, taking dodge

Lair Action
Ini17 (100,0,0), 2 4th expended, 1HP, polymorphed to Cloud Giant 190HP
Ini15a (100,0,40) 1 4th expended, 4 sorcery points expended, polymorphing Ini15a and Ini11, Young Blue Dragon 
Ini11 (100,-40,80) away 9HP, polymorphed as Young Blue Dragon
You (100,40,240) (2/3 legendary resistances) (303 HP)
Ini2 (300,0,0)

Legendary before Ini11?

No.

Ini11 approaches towards the black dragon, though keeping some distance and taking a dodge

Lair Action
Ini17 (100,0,0), 2 4th expended, 1HP, polymorphed to Cloud Giant 190HP
Ini15a (100,0,40) 1 4th expended, 4 sorcery points expended, polymorphing Ini15a and Ini11, Young Blue Dragon 
Ini11 (100,40,80) away 9HP, polymorphed as Young Blue Dragon
You (100,40,240) (2/3 legendary resistances) (303 HP)
Ini2 (300,0,0)

your turn

I Fly 160 straight down, using the dash action.

Ini2, finally away from fright, uses polymorph to be a Bone Devil and heads 40ft towards the fray (or, if we want to go back to RAW with the polymorph spell, he'll go for giant eagle and fly 80ft)

Legendary action?

*lets say he’s a bone devil*

fly 40 feet toward ini15a.

Alright.

Lair Action
Ini17 (100,0,0), 2 4th expended, 1HP, polymorphed to Cloud Giant 190HP
Ini15a (100,0,40) 1 4th expended, 4 sorcery points expended, polymorphing Ini15a and Ini11, Young Blue Dragon 
Ini11 (100,40,80) away 9HP, polymorphed as Young Blue Dragon
You (100,0,80) (2/3 legendary resistances) (303 HP)
Ini2 (260,0,0) 1 4th spell expended, polymorphed as Bone Devil

lair action?