The Gauntlet is a solo minigame in Prifddinas, unlocked after the Song of the Elves quest. Each run drops you into a randomly generated instance with nothing but your stats. You gather raw materials inside the instance, craft your own weapons, armour and food from scratch, then fight the boss at the centre. There are two versions: the normal Gauntlet and the Corrupted Gauntlet, a harder variant with worse starting conditions and better rewards.
The prep loop is half the fight. You mine crystal shards and ore, chop crystal and phren bark, and fish paddlefish for food, then use those resources to build a tier of weapon and armour and cook your supplies. Manage the timer and your resources well and you walk into the boss fully kitted. Rush it and you fight the Hunllef undergeared.
The boss is the Crystalline Hunllef (Corrupted Hunllef in the corrupted version), and it tests two things at once. It alternates between Magic and Ranged attacks, so you flick the matching protection prayer each time it switches. Separately, it builds a damage cap against whatever combat style you are using, so you bring two attack styles, usually a halberd and the bow, and swap between them to keep your damage from being throttled. Tornadoes spawn during the fight and must be walked away from while you keep this rotation going.
The Hunllef drops the crystal armour seed and the enhanced crystal weapon seed, used to make crystal armour, the Blade of saeldor and the Bow of faerdhinen. The Corrupted Gauntlet is a real step up in difficulty since you cannot fall back on normal-world resources, but its drop rates are better, making it the more efficient grind once you can clear it reliably. Run the normal version first to learn the mechanics, then move to corrupted when deaths stop being about execution.