League of Legends Eternals List is a set of premium achievements. Each champion will have a set of quests to complete for a special cosmetic. Currently Eternals are being tested on the PBE.
The Eternals will track your progress with a champion, and each champion has two Eternals sets, one common and one specific to that champion. Common Eternals will cost 225 RP or 3500 Blue Essence, and champion specific Eternals will cost 590 RP.
There are six types of Eternals: Empress, Guide, Protector, Trickster, Warden and Warrior. The Empress represents skill, Guide values opening paths, Protector prioritizes survival, Trickster rewards mind games, Warden values controlling the flow of battle and the Warrior wants to crush enemies.
League of Legends Eternals List
Here is a list of all by Champion, according to the League of Legends Wiki. The challenges for each hero are very specific, and will call on some expertise to achieve.