I will not backport this mod, it will be in 1.21.1 and potentially in later versions.
The above still remains true even with the news that ISS will support 1.21 and 1.20

If there are issues with this mod, please bring them to my player showcase thread in the Iron's Spells 'n Spellbooks discord.

Also my name is Fire, this addon does not relate to the Fire spell class in any way.

This mod currently adds 10 new Ender spells, 2 structures, 3 curios, an ender mage NPC, and a new set of armor:

SPELL LIST:

Arcane Slice: A raycast attack that deals increasing damage to the target based on how low their mana is (as a percentage of their maximum)
Aspect of the Shulker: An effect giver spell that, while active, spawns a homing shulker bullet to attack something when you hit it with magic damage.
Dimensional Adaptation: A simple QoL spell that gives you a different effect based on which dimension you are in, designed to help you survive more effectively. It will be locked, until you find a certain item. See additional notes for more info
Obsidian Rods: A continuous cast that launches long obsidian rods into a target, dealing low damage but spatially anchoring them, preventing them from using teleportation spells.
Dragon's Fury: A high knockback medium damage melee spell for the ender class that lets you make distance if being rushed.
Binary Stars: A double-cast spell that will launch two stars, one which applies a spellcasting debuff and one for melee
Scintillating Stride: A dash spell that can be casted again to do a burst of damage and return to the initial location.
Gate of Ender: A continous casting attack that summons portals around you from which sword projectiles launch in the direction you're looking. Also can be changed to a Targeted form by looking at an entity while casting. Locked behind a curio.
Hollow Crystal: A unique attack that has scaling recasts based on spell level, and every time a recast is completed (long cast), the attack that will fire off when the recasting ends becomes stronger.
Manfiest Domain: Void: A spell only obtained by crafting Singularity (void_staff), this spell has a fixed duration and will drag everyone nearby into an effectively endless empty location and give them levitation for the duration of it. Afterwards, everyone is returned to their previous locations.

Additional Notes:

The Hollow Crystal spell has a unique interaction if it collides with the Esoteric Edge from Discerning the Eldritch.

Infused Obsidian Fragments can be obtained by casting the Obsidian Rods spell and making the resulting projectiles collide with Ancient Debris
Singularity can be obtained by holding the Ornate Empty Staff and right clicking near a black hole that you have created
The Hollow Crystal Heart can be obtained by throwing an end stone and a stabilized core of ender on the ground and hitting the two with a Hollow Crystal spell

Credits and thanks:

Thank you to AceTheExaltedKing and EnderTheGuy for lots of coding help!
Thank you to Warphan for creating all of the spell icons!
Thank you to Miss Moss and M for help with the gallery photos!
Thank you to Hazen for making the End Lord armor model!

Thanks to everyone in the ISS discord who helped me with random problems I had and spell design advice!
Thank you to the MM3 server for inspiring me to make this mod <3

Fire's Ender Expansion

Ace's Spell Utils

Discerning The Eldritch

Iron's Spells 'n Spellbooks