Eternals are one of League of Legends' newest additions. It's an upgrade to the current champion mastery system that's designed to highlight progress and accomplishment more than before. This upgrade is integrated into the Champion Mastery system, which has replaced the old free stat tracker function. Eternals and the Champion Mastery system are essentially a modern reworked stat tracker that is included in nowadays League of Legends.
In the lore of League of Legends, Eternals are deities that have been lost in the passage of time. However, they've been preserved in the universe by the celestial realms, and are represented by stars and constellations. In layman's terms, Eternals are literal Gods whose true forms are no longer tangible, but can still be observed. The observation of such Gods is the cosmetic icons displayed when achieving milestones.
Each champion has a set of three Eternals, which track the player's eternal achievements. Such achievements include but are not limited to damage dealt, damage taken, takedowns, objectives taken, etc. Eternals are tracked in-game, they are also shown in-game, in the loading screen, and in the champion collection tab.
There are milestones for every Eternal. When they are achieved, you are notified in-game. An Eternal with all five completed milestones can be "reforged" into a new prestige milestone that records the Personal Best within a match. When completing 15 total milestones, or five complete milestones for each Eternal, the champion mastery emote can be upgraded.
The only Eternals available for BE are the Starter Series Eternals, which are generic Eternals. These Eternals only track Epic Monsters killed, Towers destroyed, and Takedowns. The champion-specific Eternals are the Series 1 Eternals. Only available for purchase via Riot Points, these track more nuanced statistics such as abilities hit, distance traveled by an ability, damage tanked, etc. The Starter Series Eternals are available for 225 RP or 2500 BE. The Series 1 Eternals are available only for 600 RP. To buy Eternals for every available and upcoming champion, Starter Series Passes and Series 1 Passes are also available for purchase for 5850 RP and 1350 RP or 14750 BE, respectively.
Here are some examples of champions' Eternals:
Aatrox: Worlds Ended - Enemy champions killed during World Ender (R)
Ahri: Fatal Flirtations - Charms (E) leading to kills on enemy champions within 5 seconds
Aurelion Sol: Galaxies Landed - Enemy champions stunned by Starsurge (Q)
Garen: Shields Up! - Damage mitigated with the shield component of Courage (W)
Fiora: Challenges Met - Number of Grand Challenges (R) fully triggered (All four vitals struck)
Kassadin: 404's - Channels interrupted on enemy champions with Null Sphere (Q)
Kled: Chaaaaaaaarged - Hits on enemy champions with max-wind up Chaaaaaaaarge!!! (R)
Lucian: Ashes and Dust - Enemy champions killed with The Culling ®
Malphite: In a Landslide - Multiple (2+) enemy champions hit by Unstoppable Force
Neeko: Re-Peat Blooms - Number of times all three bursts of Blooming Burst (Q) hit the same enemy champion
Ornn: Crumbles - Number of times Temper (P) has been triggered on enemy champions affected by Brittle
Singed: Fling Fiend - Enemy champions tossed with Fling (E) that land in Mega Adhesive (W)
Teemo: Shrooms Tripped - Enemy champions hit with Noxious Trap ®
Vi: Wrecking Balls - Enemy champions hit by maximum range Vault Breakers (Q)
Zed: Find My Shadow - Seconds removed from Living Shadow's (W) cooldown by hitting enemy champions with Shadow Slash (E)
Zoe: Paddle Ball - Champion Enemy champions hit by Paddle Stars (Q) that deal more than 175% of their base damage due to distance traveled
As can be seen, certain Eternals track different aspects of League of Legends as achievements. To make categorizing Eternals easier, Eternals have types, which are associated with a champion's class, and distinctive in their cosmetic appearance. The icons for each type are manifestations of the deities that Runeterreans worship.
"We're not even playing the same game."
Trickster: Deceitful mind games or premeditated chaos
"What's conflict without a little chaos?"