Ancient Relic is unlocked at lvl90.
In this game mode you can earn resources for your Bond Value, unlock an extra 4 Hidden Skill and get some friendship card packs.
This mode is about exploring floors that have different Enemies, Rewards and the stairs to the next floor, all hidden behind squares.
There are 1500 Floors in total. Every 50 Floors the enemies that you face become stronger, but also the rewards that you obtain are much better.
With all the Relic Coins that you obtain in this game mode you can exchange them in the Relic Shop for items to: Increase Bond Value, Unlock Bond Value and Unlock the extra 4 Hidden Skill Slots.