Previously you only had the Ancient and Gild system to customize your game. Gilds give bonus damage to individual heroes, and Ancients give a multitude of benefits depending on how you decide to spend your Hero Souls. These are still in the game, but are now supplemented by the Relic system.
Relics are dropped by a monster named the Relic Ooze. The Relic Ooze can spawn after the 99th floor, and only after your first ascension. You can only obtain one relic per ascension.
Each one has its own rarity and individual benefits. The higher the rarity (common, uncommon, rare, and so on), the bigger the benefit of the Relic. For instance, I’ve gotten an uncommon Runeblade of Freedom, which gives +75% gold when idle, +25% gold from chests, +1 second to Golden Clicks, and +15% gold during Golden Clicks.
As Relics can only be obtained ones per ascension, it may take a while to get four total – and it will definitely take some time to get some that give useful benefits based on your Ancient spec.
I’d been wondering what the treasure chest tab in-game was for some time, and I’m glad to see it’s turned out to be yet another unique feature to keep me coming back to Clicker Heroes.