Riot Games announced the upcoming release of a new achievement-tracking system, soon to appear on the Public Beta Environment for Patch 9.17. The Eternals system, as outlined by their /Dev blog update, is meant to be a new progressions system alongside Champion Mastery and Ranked, rewarding players with in-game milestone announcements and modified Mastery emotes for successfully accomplishing related objectives.
Notably, access to Eternals is unlocked via RP, meaning it takes real money to unlock. Per Product Manager Adriaan Noordzij's explanation:
The system has a significant amount of new tech powering it to ensure that it works consistently and reliably. We also want to support and expand upon this feature for years to come, and with so many champions to build and maintain Eternals for, the only way we could justify supporting the feature was to attach an RP price to it.
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They intend it to be similar to existing systems like Counter-Strike's StatTrak weapons or Dota 2's subscription model. Initially, players will only have access to a set of three Eternals per purchase, out of a total of six initially available and showcased in the blogpost.
Riot's preemptively addressed concerns that an achievements system may detract from serious gameplay, a potential problem amplified in a game like League of Legends that relies heavily on team synergies, stating that each were designed with followup play tests to ensure that they reward otherwise natural behaviors for that particular champion or class.
For the most part, other games with similar team-based systems and achievement trackers are largely silent on the matter. Chasing after achievements while chasing after a higher MMR, after all, usually comes at the cost of the latter.