Unknown

Unknown Roar: a devastating roar that can be heard 1000ft away. In combat This roar can push players at a range of 30-50ft back 10ft. if players are within 10-20ft range of the unknown they'll take 4d4 damage and will have to make a Constitution saving throw of 18. 

Ether blast: a multi blast attack that can hit up to 6 players in a 20ft wide sphere radius attack this attack never misses, cannot be countered. it can be dodged but players will have to a dex throw of 17. since its a multi attack you'll have to roll 6d12s (1d12 per person) 

Ether Breath attack: Entirely up to the DM on how they want to use this attack much like a dragon's breath attack you can either use a straight line breath attack that's 20ft wide and hits up to 100ft dealing 5d10s. or a cone attack that's 30ft but only hits 20ft dealing 6d12s

Spellcasting. This monster can easily cast 9th level magic because of this I would recommend that DMs mainly use this for counter spells 

Unknown Claw attack: multi attack that's caused by 6d6s slicing or piercing damage DMs choice 

Unknown Jaw attack: Warning for DM use only this is an instant kill attack. don't use on Pcs (unless they're a jerk) 

Action Ranged Attack.: Armor missiles: Unknown unleashes its powerful scales and launches it with its scales able to pierce any creature deal 3d8s piercing dmg +str and Dex modifiers 

Tail Swipe/grapple: when fighting short range the Unknown (DM) will choose either to strike a PC with its tail knocking them back 20ft making 1d12 bludgeoning damage. 

Now in my campaign my monster wouldn't normally do this attack mainly because this monster will only test out the abilities of the PCs if they survive this Encounter which hopefully your a merciful DM but hey what you want to use this monster is entirely up to you. Now when you want to use this action or attack it must be when your PCs are ready to take challenge rating 24 and up as this action will pretty much be a board wipe covering 200ft think it as a supersized fireball made of Ether. 

Unknown Meteor: 3 rounds to charge as an action. when released players will be automatically be brought to zero if players were already knocked uncounsience before this they must make an additional saving throw to see if they live or not if they fail; their characters are automatically disintregrated; player who get hit uncounsience from this dont need to make a saving throw for the surviving players they'll be brought back to one hit point and receive their EXP points for they have succeded their encounter by the unknown.