Cosmic Focus

Legendary Wondrous Item (Spellcasting Focus), Requires Attunement

Can be used as a spellcasting focus. While attuned to this item, you gain a +3 bonus to the spell attack rolls and the saving throw DCs of your spells.
Doubles the range of all teleportation spells cast through it.

Universal Portal. As an action, you can telekinetically move this focus within 30 feet of you and transform it into a semi-permanent teleportation circle that will, if you wish so, automatically create a portal to a different permanent teleportation circle of your choice that will appear after 2 full rounds and last for 1 round. It functions as a regular permanent teleportation circle, but while attuned to it, no matter how far it is, you can use your action to transform it back into its spellcasting focus and teleport it to you.

Unlike other Teleportation Circles, Cosmic Focus can create a portal to different planes of existence
While the Cosmic Focus serves as a portal, you lose the ability to use any of its other features.

Gateway of the Void. If you cast a spell that summons creatures through this item, its effects works as if you cast it using a spell slot of 1 level higher, and the summoned creatures gain additional effects and bonuses

Their attacks deal extra force damage equal to your spellcasting modifier
Each time you use a teleportation spell, you can also make it target all of your summoned creatures as well
You can cast spells as if you were in your summoned creature’s space
You can use your bonus action to swap places with your summoned creature within 90 feet of you.

Dimension Step. Each time you cast a spell or if an attack misses you, you can automatically teleport up to 10 feet away to an unoccupied space.

Cosmic Prison. You know the Banishment spell while attuned to the focus, and can cast it once per minute at 4th level without expending a spell slot.