How the encounter most likely goes down (bearing in mind during the following the PC's by this stage have dealt with 6 encounters including against an Archmage, and a spellcasting bunch of Githzeri. Resources will be very low by this last encounter):
The Atropal is hiding at the far end of the room behind a Pyramid (on top of the pyramid lies the +3 vorpal sword). It has a stealth score of +11 (thanks to the shadow template). The gloom grants it advantage on its stealth checks. Its perception score is +12 (passive 22), and it has blindsight to 60'.
Assuming the PCs (and
Celtavian and
Azurewraiths parties both use this tactic by default) advance in a tight group, the Atropal most likely notices them first. Grant the PC's and the Atropal opposed perception vs stealth checks at 120' (everyone gets advantage to stealth due to the dim light). In any event, the Atropal automatically notices the PC's when they reach 60' away from the pyramid thanks to blindsight (the PCs no longer have any shadows or gloom to hide 'in', and the Atropal looks right at them).
The most likely outcome is several PC's will be surprised on round 1, and the encoutner starts at a range of 120' with the Atropal in 3/4 cover and hidden, and the PCs caught in the open.
The instant the atropal (or PCs) notice one another, combat is initiatied and initiative rolled.
On
round one (assuming the atropal is aware of the PC's and not surprised) at initaitive count 20 the Atropal spams its first lair action [20' radius, 120' range] catching all the PCs in writing tendrils of shadow. The area the PCs are in becomes difficult terrain, and they all need to make DC 15 strength vs being restrained.
Remember, restrained creatures:
- The creature's speed becomes 0, and it cannot benefit from bonuses to its speed.
- Attacks against the creature have advantage, and the creature's attacks have disadvantage.
- The creature has disadvantage on Dexterity saving throws.
If a PC rolled a higher inititiative score, after that PCs turn (the PC likely spends that turn doing nothing as they are surprised) it immediately uses a legendary action to detect - scanning for any PC's it hasnt yet seen or who are hidden. It holds the rest of its legendary actions in reserve for later on in round one.
On its turn, it flies 80' forwards (hovering about 30' up in the air) and uses its fightful presence on any PC aware of it, and then breathes - catching all the PCs who are grouped together, and also likely restrained. (Remember - any restrained PC's also get disadvantage on the DC18 Dex saving throw against this attack). It most likely deals 16d6 (around 55 points on average) necrotic damage to all the PCs at this point (necrotic damage, so its unlikely to be resisted). As it is breathing from hiding, even PCs that are no longer surprised and able to take reactions such as uncanny dodge and feral reflexes and similar abilities can't use them (the PC's cant see the atropal until after the attack is resolved).
DC 27 Concentration checks (on average) are now required for any PCs that have any concentration buffs active prior to entering the room.
If any PC somehow closes to melee with the atropal in the surprise round (via flight and not being restrained via the lair ability), it uses its remaining legendary actions to use its wing buffet ability after that PCs turn, forcing that PC to make a DC 19 dex save or be knocked prone (and thus fall) taking bludgeoning damage from the buffet, and from the fall. The dragon then moves 40' away as part of the same action.
Round one now comes to an end.
On
round two, it spams a 60' long, 10' high, 5' thick wall of shadowy spiked dark matter on initative count 20 via a lair action. Creatures in the area make DC 15 save vs 4d8 piercing (half damage on a success). This likely splits up the party, and pushes PCs outside the range of any Paladin auras.
It hovers 70' in the air and uses its wing buffet legendary action on any PC that closes to it while it waits its turn on round 2 (DC19 to knock them to the ground). It relies its legendary resistances and resistance to all damage barring force, psychic and radiant while it assesses the PCs actions and determines who poses the greatest threat via being able to hurt it (likely a spell caster or ranged PC).
When its turn comes around it uses frightful presence on anyone yet affected (they can all see it now) and checks to see if its breath recharges (using it if it does). If its breath weapon is not yet available, it flies down to a badly wounded PC (or one that hurt it prior with a ranged attack or spell) and uses multiattack (preferring to target a creature that is cut off from the other PCs via the lair action wall of thorns if possible). It strikes a downed PC one additional time forcing 2 failed death saves (knowing that this will hopefully draw a PC to his downed comrade the following turn).
Following its turn (its now on the ground) it again uses a legendary action following the next PC's turn to use wing buffet again (to avoid getting surrounded in melee by knocking people prone, and then flying away). This likely kills the downed PC (by inflicting a third failed death save). It then flies 40' upwards and waits.
Round two ends.
It starts
round 3 most likely back in the air hovering and out of the reach of PCs, and it uses its lair action on initiative count 20 to
Charm a PC (preferring to target an isolated or cut off PC) or to again restrain them in shadows while it hovers out of danger. It then follows its tactics from round two (again checking for breath recharge at the start of its turn, using its breath when it becomes availaible, knocking any flying PCs out of the sky with wing buffet, and relying on its resistance to most damage types, and its legendary saves to keep it alive). It continues to target any PC that can hurt it (likely a PC with a ranged weapon or a Paladin that has somehow been able to reach it and burst it with radiant damage from Divine smite).
Im totally stoked you ran it
Azurewraith, and Im glad your group had a blast, but I have a gut feeling there is no way your PCs could have survived that encounter if run like that (particularly after the resource expenditure of the prior six encounters).