Elder Elemental (Elder Tempest)

Disclaimer: This subclass is not properly balanced and as such is probably far too powerful or weak. Also, I'm not good at this so you will have to keep track of the features yourself.

Terrifying storms manifest in the body of the elder tempest. A being carved from clouds, wind, rain, and lightning, the elder tempest assumes the shape of a serpent that slithers through the sky. The tempest drowns the land beneath it with rain and stabs the earth with lances of lightning. Punishing winds scream around it as it flies, feeding the chaos it creates. The Elder Tempest is a powerful being from the Elemental Plane of Air that has utterly ravaged the palace of many a Djinn are they are powerless to resist. If they were to reach the material plane they would destroy the people there as well. They bring their power to the material plane through prospective warlocks seeking power themselves. These warlocks are often extremely dedicated and will commonly band together with others to form cults that attempt to summon the great one who rules the skies.

Flying speed

You gain a flying speed equal to your walking speed.

Bonus cantrip

You gain the gust and booming blade cantrips at level 1.

Eye of the storm

From 6th level, your affinity with the Elder Tempest gives you resistance to thunder and lightning damage.

Tornado

At 6th level, you can summon a tornado. All creatures in a 5-by-30-foot line in front of you must make a dexterity saving throw or take 2d10 bludgeoning damage and 1d10 thunder damage.

Lightning storm

At 10th level, a number of times equal to half of your warlock level rounded down, you can force all creatures in a 60-foot radius to make a dexterity saving throw or take 6d8 lightning on a failed save or half as much on a success. All of the creatures in this range must also make a constitution saving throw or be stunned until the end of your next turn.

Storm caller

You gain the conjure elemental spell when you reach 10th level but you can only summon an air elemental.

Walking on air

Once per short rest, you can cast wind walk on yourself.

Ethereal morph

Once per short rest, you can enter the ethereal plane for any length of time. When leaving the ethereal plane you make a wisdom saving throw. On a failure, you take 6d6 psychic damage.