Empyrean Traces and Hidden Cubes

In short

Empyrean Traces (players call them "feathers") are the glowing pickups scattered over every zone, on the ground, in the air and in water. You deliver them to your zone's Monolith to level it.
Monolith levels are worth it: permanent stats, every Revelation Amulet scroll your amulet needs, Hidden Cube keys, and the Abyss Monolith gives PvP stats plus Azphel board crystals. Pick up every Trace you fly past.
Hidden Cubes are treasure cubes hidden around the map. Open them whenever you find them, at any level: their loot is the same at level 10 and 45 (the game files have no level scaling for them).
A key makes a cube much better (for example 1,000 Soul Crystals instead of 100). Some cubes only open with a key.
The "wait until 45" advice is about dungeon cubes, the loot cube after a dungeon boss. Those cost Odyle Energy, so skip them while leveling and spend the energy at 45. Energy keeps banking up to 840, so you lose nothing by waiting.

Empyrean Traces and Monoliths

A glowing Empyrean Trace feather standing in the grass in Verteron — An Empyrean Trace in Verteron. Press F next to it to collect; the minimap also marks them.

Verteron Monolith window at level 2 with resonance effects and level rewards — The Verteron Monolith at level 2: Traces delivered, stat effects per level and the next rewards.

Monolith window with the Sync Empyrean Traces button bottom left — The Sync Empyrean Traces button (bottom left of the Monolith window) hands in every Trace you carry.

Each faction has three Monoliths, one per region, and each takes its own Trace:

Monolith (Elyos / Asmodian)	Trace	Max level	Traces to max	What it gives
Verteron / Altgard	Empyrean Trace: Verteron (or Altgard)	30	560	Empyrean Lord stats, Revelation Amulet scrolls (30), Hidden Cube keys (12), Soul Codex resets (18), Unique Amplify Stone Fragments (18), 3 titles
Eltnen / Morheim	Empyrean Trace: Eltnen (or Morheim)	8	45	Max Flight +7,000, Empyrean Lord stats, amulet scrolls (5), Hidden Cube keys (15), Soul Codex resets (6)
Reshanta (the Abyss)	Empyrean Trace: Reshanta	20	300	PvP stats, Daevanion Crystal: Azphel (52), Unique Amplify Stone Fragments (36), 2 titles

"Traces to max" is the running total the Monolith asks for, not per level.

Empyrean Lord stats are the same family the Arcana cards give (Life, Destiny, Time, Space, Justice, Destruction, Freedom, Death, Illusion, Wisdom). Verteron raises each by 5 or 6.
The amulet chain runs on these scrolls. Your PvE amulet (Amulet of Revelation) upgrades with Revelation Amulet scrolls, and the Verteron Monolith hands out one per level. See belt and amulet.
The Abyss Monolith gives PvP Damage Tolerance, PvP Accuracy, Evasion, Block and PvP damage, plus crystals for the Azphel (PvP) Daevanion board. Creators report it's capped at level 16 on Global for now, which is 200 Traces and 36 Azphel crystals.

Every Monolith level and its reward (Elyos names; Asmodian Monoliths are the same)

Verteron (each level also gives 1 Revelation Amulet scroll, 2 at level 30)

Level	Total Traces	Stats	Item
2	1	Life, Destiny +1	Hidden Cube key ×1
3	5	Time, Space +1	Hidden Cube key ×1
4	11	Justice, Destruction +1	Hidden Cube key ×1
5	20	Freedom, Death +1	Hidden Cube key ×1
6-9	30 / 40 / 50 / 60	the same pairs again	Hidden Cube key ×2 each
10	70	Illusion, Wisdom +1	Title: Verteron Inquirer
11-19	80 to 215	the pairs again	Soul Codex: Reset ×1 to ×3
20	235	Illusion, Wisdom +1	Title: Guardian of Verteron
21-29	255 to 510	the pairs again	Unique Amplify Stone Fragment ×1 to ×3
30	560	Illusion, Wisdom +1	Title: Saint of Verteron

Eltnen (every level also gives Max Flight +1,000)

Level	Total Traces	Stats	Items
2	5	Life, Destiny +1	Amulet scroll, Hidden Cube key ×3
3	10	Life, Destiny +1	Soul Codex: Reset ×3
4	15	Life, Destiny +3	Amulet scroll, Hidden Cube key ×3
5	20	Time, Space +1	Soul Codex: Reset ×3
6	25	Time, Space +1	Amulet scroll, Hidden Cube key ×3
7	35	Time, Space +3	Amulet scroll, Hidden Cube key ×3
8	45	Justice, Destruction +5	Amulet scroll, Hidden Cube key ×3

Reshanta (every level gives +15 PvP Damage Tolerance, or +5 PvP damage on levels 5, 10, 15 and 20, plus +5 PvP Accuracy, Evasion or Block)

Levels	Total Traces	Azphel crystals per level	Other
2-5	1 / 5 / 11 / 21	1	Unique Amplify Stone Fragment ×1
6-9	31 / 43 / 55 / 70	2	Fragment ×2
10	85	3	Title: Guardian of Ereshkigal
11-13	100 / 120 / 140	3	Fragment ×2
14-16	160 / 180 / 200	4	Fragment ×2-3
17-19	225 / 250 / 275	4	Fragment ×3-4
20	300	4	Title: Saint of Ereshkigal

Hidden Cubes

Hidden Cubes are treasure cubes hidden around the open world. Three kinds show up:

Normal cubes open by hand. A key is optional and makes the loot better ("Use a key to get better rewards").
Key-only cubes don't open without a key ("Cannot open without a key").

Hidden Cube prompt with a Key: Hidden Cube slot and the Open with Key button — Opening a key-only cube: the key slot, "Cannot open without a key" and Open with Key.

Event cubes during field events, with their own loot.

Does your level matter?

No. In the game files, field Hidden Cube loot is one fixed table with no level scaling, and players on Reddit say the same ("You can open at any level and the rewards are the same"). The only reason to wait is that it's easier to hunt cubes once you can fly freely at 45.

What's inside

Every cube also gives Silentium (1 without a key, 10 with one) and sometimes Small Odyle Energy. Then it rolls one prize:

Prize	Without a key	With a key
Soul Crystals (Bound), the wildcard pet crystals	×100 (about 1 in 3)	×1,000 (about 1 in 6)
Soul Codex (Bound)	×1 (about 1 in 4)	×5 (about 1 in 6)
Power Shards or Superior Power Shards	50-100 (about 1 in 4)	100-200 (about 1 in 6)
Seed of Detection		×1 (about 1 in 6)
Mana Potions or Life Crystals	1-2 (about 1 in 9)	3-5 (about 1 in 12)
Resurrection Spiritstone	rare	about 1 in 12
Status Effect Resist Scroll	rare	rare
Lucky Aura (Rare, Epic or Unique)	rare	rare
Shugo Transformation Candy	rare	rare
Soul Codex: Reset	very rare	rare
Cosmetics: helm and jewelry skins, Radiant Love Wings, Emote: Kurukuru	very rare	very rare

Where keys come from

Monoliths: Verteron levels 2-9 (12 keys) and Eltnen levels 2-8 (15 keys).
Duty quests: pick the cube key when it's among the 5 random rewards (Grobs, FRESHY).
Quests, achievements, attendance, the Daeva Pass and the season coin shop.
Achievements count cubes too: open 3 to 200 cubes, and 3 to 50 with a key, for rewards.

Dungeon cubes: the ones to save for 45

The loot cube after a dungeon boss is a different thing. Opening it costs Odyle Energy: 20-30 on Easy, 40 on most dungeons, 60 on a few hard ones.

Energy isn't lost by waiting. It regenerates 120 a day and banks up to 840 (7 days), from level 22. See Odyle Energy.
Watch the unopened-cube limit at 45: in Conquest and Transcendence, 10 runs where you skip the cube lock you out of that dungeon until the weekly reset (aLuckyRO, Spid). Do those runs right after reset, or open a cube on one of them.

Quick plan

While leveling: grab every Empyrean Trace and level the Monoliths. Open every Hidden Cube you find. Skip dungeon loot cubes.
Use keys on key-only cubes first, then on normal ones.
At 45: max the Verteron Monolith for the amulet scrolls, keep feeding the Abyss Monolith, and start spending Odyle Energy on Conquest cubes.