Hydra lamps is a Treasure Hunter promotion, originally released in March 2017. During it some of the regular skill and prismatic prizes are replaced with hydra lamps.
Hydra lamps allow the player to select 2 skills to gain experience in as opposed to just a single skill while also using some bonus experience in those selected skills.
Hydra lamps are similar to smouldering lamps, giving the player regular experience in a skill and immediately awarding up to the same amount of the player's stored bonus experience. However the lamp awards experience and (where applicable) bonus experience in two skills rather than one. Each skill receives 60% of the total experience it would earn from a smouldering lamp which is 120% in total.
Since the promotion's appearance in the Advent Calendar promotion on 4 December 2020, opening any chest offers three rewards to choose from. The first offered reward is the main reward from the opened chest while the remaining two are generally the same rarity and category as the first one.