Since part of this went live before it was meant to due to fixing a progression stopping bug, and I've already gotten an email asking about what the equipment slot that appeared is and why they can't find something to equip there, I figured I'd go into what these are going to be.
Relics are a version of the Djinn system from Golden Sun. For anyone that's not sure what that is, in Golden Sun and the two follow up games there are creatures called Djinn that you find as you progress through the game and can equip to a character. Each is tied to one of four elements and come with a skill that ranges from damage to support. Using that skill puts them in a "set" state, or you can manually do this outside of battle, and when enough of certain types are in that state you are able to use powerful damaging skills. The other thing they do is change the class of the character using them based on how many of each element you equip to that character, but as I went with the pokemon style of collecting multiple characters rather than a class swap system, this isn't part of my system. (Though it was a very cool way to have multiple classes with only 4 characters in the first game and most of the second)
The reason it's taken so long for these to be added, as they were originally planned to be introduced in the base game and more added over time, is that I couldn't get them working the way I originally wanted. I've been working on various options, from a more Djinn like system where you equip a bunch that essentially let you "make your own sub-class" on whatever party you pick, to a more interactive system with special cut in effects and the like, and the results ranged from unimpressive bloat to causing the game to crash due to expecting far too much out of the engine. In the end I settled on the aspect that most people would remember from the Djinn system and focused on the summons part. While I can't do anything as impressive as a 3D animation of summing a dragon in space that launches a beam of energy to wipe out your foes, or a giant armored figure that summons a wave of devastation, or a planet sized sun goddess that calls down the might of a star on your enemies, and I so very much wanted to as that was the original plan, what I ended up with was something a bit more useful than just "massive damage."
Each character can only equip one Relic at a time, but each grants you an ability or a passive that wouldn't otherwise be available on a normal skill. These abilities have no cost outside of a hefty cooldown to make sure you can't spam them, cannot miss, and have the fastest cast time in the game so they (should) always go first, letting you play around the effect of the skill used. Obtaining them starts once you begin Act 2 and will range from finding where some are hidden, solving a puzzle, or fighting a boss level enemy at the power level of the new content. (level 50+ is going to be recommended for that) They'll be scattered throughout the game, so you have a reason to go back to places like Hin Su or start a fresh play through to see the other side of the core story if you only played it once. The more you collect, the easier it will be to get more so the snowball concept continues.
Here are some examples to showcase what you can expect.
Ornate Timepiece - Cooldowns for the equipped character are lowered by 2 per turn instead of 1. (Passive, so always on)
Great Serpent's Fang - Grants a skill that damages and applies the Venom DoT effect to one foe. Venom lasts for between 8 to 16 turns and deals 12% enemy health per turn. (For reference. Bleed lasts for between 2 and 4 turns per application and deals 10% enemy health per turn.)
Hammer of the Flames - Grants a skill that damages and applies the Burn and "Melted Armor" states to all foes. Melted Armor negates 100% of enemy defenses, lowering by 20% per turn until it expires.
Coiling Darkness - Grants a skill that deals damage and marks a foe. For the next 4 turns, all damage the character that cast this skill deals to the enemy is tracked. When the status expires, half of all tracked damage is then dealt to the affected foe in one burst.
As you can see there's a mix of effects. Some are passive, some are basic but still heavy damaging abilities, and others have effects I otherwise wouldn't just slap on any random skill set because of what someone like Esthera with a full Crit build can dish out. I'm fully expecting most of these to trivialize content even more than the pokemon style of balancing already does, but that's part of the reason for them existing in the first place as Djinn summons trivialized the Golden Sun games once you got past the initial set for having under 4 of the same type equipped. They won't be usable on every fight though. The 4 puzzle fights will disable all but the passive ones, because those fights are already held together only by my hopes that I never have to touch them again and these may sequence break them causing issues, and Tishna's character quest will disable them along with all other skills and weapon unleash's until you reach the end fight to get you to figure out various ways her aura's can be used. Otherwise they're fair game for the various fights across the game.
As this is the last planned content update there's going to be a few opportunities where having them will be useful as I still have the Megaman Zero "boss rush" to implement along with a few more things that they should help with clearing. So I don't scare anyone into thinking they'll need these, I'm not balancing around having any of them so you won't need them to clear the content. They'll just make it far easier to complete than it otherwise would be. So if you're struggling with something with your current team you can use one to get around needed to swap if you like your team composition.