This type of equipment is given from level 66 quests. Relics come in two types, power or crit. Higher rarity Relics also give extra defense modifier, mana points and endurance. Like Halidoms Relics follow item rarity colors. Uncommon ones are green, rares blue and superior golden (yellow).
This equipment is upgraded by using other Relics of same type and tier. In example Seren's Relic I is upgraded with Seren's Relic I. Upgrade has probability of success, that is not shown. To upgrade Seren's Relic II you'll need second piece of Seren's Relic II that is consumed on attempt.
Tier I uncommon (green) relics are monster drops or from new high level Macellarius Catacombs. Do not waste much gold upgrading green ones. Blue ones drop in Azart Hatchery or from Hazard. Sell extra green ones for gold or feed them to pets to level it up.
Relics don't follow the 2:1 crit:power ratio like most equipment does. Power relics are therefore more powerful than crit relics. It's recommended to always use power versions and build crit with equipment that can be switched to crit with 2:1 ratio. (Unless you're healer and don't need power.)
There's only one version of superior Relic which gives both power and crit (Amarun's Relic I +6 power, +11 crit, +7 endurance, 392 MP). You might want to skip this and use upgraded blue version.