Create Lair

The caster creates a 300 foot by 300 foot cubic lair of their own design in which they can choose up to 25 creatures to bring inside of it (with a minimum of 1 creature). If a creature is unwilling to enter the lair, they must succeed a DC (27 + Spellcasting modifier) Wisdom Saving throw, or be forced into the lair. 

Upon creating the spell, the caster gains +2 to armor class, +5 to saving throws, immunity to critical hits, and one of the following benefits while inside the lair:

Abjuration: Gain immunity to two damage types of your choosing until your next turn. Also gain an additional +3 to armor class.
Divination: Force a reroll a number of times equal to the caster's level. Gain the benefits of the spell Foresight.
Conjuration: Conjure two separate creatures that have up to a challenge rating of 15, which are friendly to you and individuals of your choosing.
Enchantment: Invoke any condition on any or all individuals in the realm, with the exception of exhaustion.
Evocation: Expand the area of effect of your spells up to the range distance of your lair and increase the amount of creatures you can target with a spell. You and individuals of your choosing are also immune to the effects or damage of your own spells.
Illusion: Gain the ability to cast Mirror Image, Hypnotic Pattern, Greater Invisibility, and Illusory Dragon at will. Caster gains the benefits of the Cloak of Displacement.
Necromancy: Gain the ability to create an surge of necrotic energy over the entire lair that deals 4d10 necrotic energy to any or all creatures within the lair, which grants temporary hit points to the caster equals to half of the total damage dealt to all creatures affected.
Transmutation: Change the gravity cutting the movement of those in your realm by half and dealing 6d6 bludgeoning damage. Also gain the ability to cast Reverse Gravity three times without expended a spell slot.

The caster can maintain concentration on two spells while in the lair. In addition to this, you have advantage on Constitution saving throws that you make to maintain your concentration on a spell when you take damage. The concentration DC for the caster within their own lair equals 15, regardless of the amount of damage taken. 

Nothing can teleport and physically pass through the walls of the lair as a means to leave unless the caster allows it. If a creature is not wanted inside of a caster's lair, succeeding a Wisdom Saving throw against the caster's spell save DC, allows one to enter the lair once. It is immune to all damage and can't be dispelled by Dispel Magic or countered by Counterspell. A Disintegrate and Antimagic Field spell puts a 20 foot wide hole in the wall instantly, however. The wall also extends into the Ethereal Plane, blocking ethereal Travel through the wall.

If a lair is already in play, you can suppress it for the full duration of your own by succeeding a Wisdom Saving throw against the opposing creatures spell save DC. Creating a lair this way cancels out the effects of natural lairs, such is that of a lich or ancient red dragon.

Lairs also have additional effects which can be determined by the DM and player. Such as terrain change, damage over time, or other properties.

The aesthetics of a lair usually reflects the personality and soul of the caster. A bard might have a concert of some kind that utilizes deafening sound to their advantage, a druid might create an elaborate forest or hedge maze to distract and trap victims, a wizard might create a mansion of magical runes with powerful effects, and so on.

Some examples of elemental lairs, their additional effects, and their aesthetics include: 

Flame: An ashy, volcanic or desert landscape that deals 2d6 fire damage on each turn, one level of exhaustion every third turn, and all stealth checks are made at disadvantage.
Water: A deep dark oceanic sight bears witness in your lair, anything without a swim speed moves as if in difficult terrain, all creatures take 1d6 cold damage at the start of the caster's turn, and fire attacks automatically fail and have their damage reduced to 0.
Earth: A vined forest or a rocky expanse, falling rocks deal 4d6 bludgeoning damage every other turn, difficult terrain to all on the ground and creatures in the lair lose a flight speed.
Wind: An expanse of bright blue sky, anything without a fly speed is considered falling and moves as if in difficult terrain, ranged attacks on caster have a -5 penalty to hit and disadvantage, and anything with a flight speed moves at double speed.
Spirit: An ethereal expanse that exemplifies the Ethereal Plane, once per round the caster could cast a spell as a bonus action, and all creatures have disadvantage to hit the caster with a melee attack.

If concentration is broken, or the duration of the spell ends, the caster gains two levels of exhaustion and cannot cast this spell for 48 hours. The spell also ends if the caster falls unconscious or dies during the duration of the spell. Trying to cast this spell before the 48 hours is up inflicts 8d6 psychic damage to the caster and the spell slot is used with no other effect.