Crucible is another game mode available for players in Heroes of Skyrealm.
A 1-on-1 duel/theft to loot other player’s items. This relies on the player's statistics to see who the winner will be. There are different relics to be looted from other players. Players are "stealing" items from other players through battles based on your Hero's statistics and Power.
Tapping the blue sword icon places you into an auto duel, while the green sword icon is straight on theft from what I can tell.
Afterwards, you can combine what you’ve looted into relics that you can equip. You have two choices here.
Each round of looting will cost 3 Vigor.