What is thunderlords decree?

Thunderlord's Decree is a keystone mastery in League of Legends that can be selected in the Cunning tree. It is used primarily by damage-dealing champions like assassins and mages. Thunderlord's Decree provides burst damage and helps champions secure kills.

When a champion damages an enemy three times within three seconds with Thunderlord's Decree, the mastery will trigger and deal bonus magic damage to the target and other nearby enemies. The bonus damage scales with the champion's level and attack damage or ability power.

Thunderlord's Decree was very popular in the game's earlier seasons, especially in mid lane, until it was eventually replaced by other keystone masteries like Electrocute and Dark Harvest.