No matter your level, the Ruins mode always offers valuable rewards. This page of the guide explains everything about the Ruins mode.
How to unlock Ruins?
To start the Ruins mode you must first enter the Adventure menu (upper-right corner, crossed swords icon), then the Recommended tab. You'll learn more about the Adventure menu on a separate page. You must be at least level 11 to access the challenge.
Entering the Ruins tab displays a menu, which lets you select any floor of the Ruins. If this is your first time visiting one of them, you must first find them out in the game world by heading to the location shown on the map.
After choosing the Ruins you can select the difficulty on the right side of the screen. Make sure you meet the required character level. You should meet the recommended CS as well. If you don't, you should spend some time upgrading your gear before beginning the challenge.
The Ruins can only be played through in singleplayer. You can't call any other players for help, though at least there is no time limit either.
At first, the mode pits you against singular enemies or smaller groups. After the encounter you'll face a puzzle. These usually vary depending on the floor you're on, most often requiring the use of Relics.
One of the puzzles may be a wall of lasers blocking the way forward. Trying to pass through them knocks you back and deals heavy damage. To get through, you must use the Omnium Handcannon relic to create a column and block the lasers. This lets you get through to the other side.
After reaching the end of the ruins you'll face a boss battle. These only get challenging on higher difficulties, but you should still be mindful, especially when the boss uses attacks telegraphed with red. Perfect dodging will grant you charges needed to use your weapon's special skill.
The rewards for succeeding in this mode vary. Usually, you'll receive a large amount of experience, gold and relic shards used to upgrade the Relics. You can also acquire Potent Omnium Crystals used to upgrade your Suppressor.