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It’s my turn, right?

Ini11 moves 80ft and attempts to grapple Ini2 (to stop him fleeing so far)

Ini11's 17 vs Ini2's 4 (STR vs DEX)

using x,y,z for coordinate grid now. gonna set Ini17's current location as (0,0,0) (because he's on the ground and I don't like the idea of ground level being z = -80)

Lair Action
Ini17 (0,0,0), 2 4th expended, 1HP, polymorphed to Cloud Giant 200HP
Ini15a (140,0,0) 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 (2/3 legendary resistances) (282 HP)
Ini2 (140,0,0) (frightened)

Your turn

and you're at (0,0,40) (not 80 since I also forgot you'd used your legendary actions)

Breath weapon recharge:3

I continue flying upwards, using the dash action to bring myself 200 feet in the air.

Oh wait. I made BIG oopsie. I totally missed that polymorph is limited to Beast creature type. Bone Devil, Cloud Giant, Young Blue Dragon. None of those are beasts.

Not sure how you want to proceed here.

Quote from VitusW >>

Oh wait. I made BIG oopsie. I totally missed that polymorph is limited to Beast creature type. Bone Devil, Cloud Giant, Young Blue Dragon. None of those are beasts.

Not sure how you want to proceed here.

 Ehhh it doesn’t really matter since we already did it. Lets just ignore that part.

Okay. 
Question: What kind of terrain is your lair? Thinking if Ini17 can find any rocks.

Ini2 isn't grapped. 

WIS save

4 or 4

Lair Action
Ini17 (0,0,0), 2 4th expended, 1HP, polymorphed to Cloud Giant 200HP
Ini15a (140,0,0) 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 (200,0,0) (frightened tbd)

Lair Action Time

The lair is a mossy cave with multiple holes at the top, large enough for me to fly through. The ground is mossy, with sharp rocks sticking out of the ground, and scattered around are a lot of 20/30 foot deep pools of water.

Swarming insects appear in a 20 foot sphere around ini17. Make a DC 15 con save or take 10 poison damage. At the end of their turn, if they are still in the swarm, they take the damage again, no saving throw.

Ini17 CON save: 29 for insects

Ini17 moves out of the swarm and towards his allies (40,0,0) picking up a rock and chucking it, with disadvantage, at the dragon

To-Hit: 18 or 15
Damage: 35

That's a miss. Legendary action before Ini15a?

Yep, move 40 feet higher into the air.

wait, the insects are still half damage on save and it's 3d10 piercing, not 3d6 poison
And while it's technically within 120ft, considering I messed up big-time on the polymorph I think it's only fair to not worry about lair action range for you

Quote from VitusW >>

wait, the insects are still half damage on save and it's 3d10 piercing, not 3d6 poison
And while it's technically within 120ft, considering I messed up big-time on the polymorph I think it's only fair to not worry about lair action range for you

 Oh. I thought i had it memorized so i didn’t check.

Quote from TheFriendlyArchfey >>

Quote from VitusW >>

wait, the insects are still half damage on save and it's 3d10 piercing, not 3d6 poison
And while it's technically within 120ft, considering I messed up big-time on the polymorph I think it's only fair to not worry about lair action range for you

 Oh. I thought i had it memorized so i didn’t check.

 I took another look. I think you got 3d6 from the Filthy Burst which does Poison damage AND makes it difficult terrain.

So giant has to make end of turn save 22 against another 3d6 poison

and concentration roll: 24

and he's at (20,0,0) not (40,0,0)

Quote from VitusW >>

Quote from TheFriendlyArchfey >>

Quote from VitusW >>

wait, the insects are still half damage on save and it's 3d10 piercing, not 3d6 poison
And while it's technically within 120ft, considering I messed up big-time on the polymorph I think it's only fair to not worry about lair action range for you

 Oh. I thought i had it memorized so i didn’t check.

 I took another look. I think you got 3d6 from the Filthy Burst which does Poison damage AND makes it difficult terrain.

 Huh.