Impossible Trial 6 rewards for Gold are Crest of Surtr, Greater Crest of Flame, Perfect Sigil of the Realm, Smouldering Ember and Hacksilver. To unlock the impossible trial 6 you need to collect 3 trial keys from previous impossible trials.
One of the objectives for trial 6 is to kill enemies. To unlock the gold tier rewards you need to defeat 50 enemies in 8 minutes. Completing trial 6 will unlock the Fire and Brimstone trophy and reward you with 2 legendary chests.