What is it? Good question! An OSRS league is a temporary gamemode characterised by increased XP rates, restricted area access, and powerful 'relics' that boost your power in skilling and combat.
You get a total of five areas, and there are eight relic tiers each with 2-3 relics to pick from. Areas are cohesive sections of the game world, large enough to contain multiple bosses and skilling methods. Relics give powerful buffs like harvesting 10x as many crops as usual, or being able to teleport to any boss instantly. Additionally in this League there is a tree of Demonic Pacts that give powerful combat bonuses and allow for diverse skill combinations.
You complete various esoteric or difficult tasks for points, which can later be spent in the main game for cosmetic rewards. Realistically though I'm just doing it for the love of the game, and to see number go up.
You can find more comprehensive information on Leagues 6 on the OSRS Wiki
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