Crafted Eternals in the Arena.
you can change an Eternals Element. example: change Fire to Arcane. (Crystal Calcite)
you can change an Eternals Class. example: change Warrior to Rogue. (Crystal Jasper)
you can add a Myth Stone if the Eternal has an empty Socket. example: socket + (MS) Protest = +250 All resist.
you can change the Nature. example: change Greed to Death. (Crystal Beryl)
you can change the Talent. example: change Prismatic to Fester. (Crystal Citrine)
you can recover a Myth Stone. example: if you place (MS) Boon on your Eternal, and want to put (MS) Protest instead. (Crystal Garnet)
Note: if you used the Myth Stones to make a Mythic Affix, the Myth Stones are gone and cannot be recovered by using (CS) Garnet.
Unreal Hood & Fabled Staff/Blade: there is 1 Hood and Weapon for each class. they have 4 Sockets each. you could put 1 Myth Stone in each Socket for a total of 4 Myth Stones.
you can add a Mythic Affix on these 2 Eternals if you know the recipe. when the Mythic Affix is created...
you can add any combination of 3 Normal and/or Epic Affixes.
example: on an Unreal Hood I make the Harmony Mythic Affix. the 4 sockets are used up to make the Mythic, and leave 3 empty spaces. I can now place a random Normal/Epic affix in each space using (Crystal Angelite) or place an Epic affix using (Crystal Topaz). Angelite is a bad choice, because, unless you are very lucky, it might take forever to get the affix you want. Topaz gives 6 choices when used, so you have a better chance to get the affixes you want. either way, what ever you pick is permanent. so if you don't like the choices, you need to get a new item and start over.
Edit: I tested all of the Crystals, and these are the ones that can be used on Eternal Items. I did not test them on Eternal Pets, Trinkets, or Maps.
there is a list of Eternal Items in the Codex. if you see a modification that isn't from one of these examples, or are unsure, that is when you send a post to [Reports Only] Report Cheaters & Hackers, and a Dev will check it out. if you can, check against the Codex first, and if you are still unsure, then report it.
for those who don't know, if you use a cheat or hack to modify your Eternal Item, and a Dev becomes aware of it, you get sent to the Banned League. it is just like the Arena, except all the players are also using cheats or hacks, or are waiting for the Dev's to determine that they did/didn't use cheats or hacks, and if not guilty, get to go back to the Original League.