Season 18 has begun in Diablo 3, and this time it's themed around the Triune—a reference to an ancient cult that worshiped false forms of the Diablo universe's three Prime Evils. Back then, they showed up in the friendly forms of Dedication, Creation, and Love, and the idea was that they could tilt the balance in the war against Heaven in their favor by hoodwinking humans into thinking they were beneficent deities.
And sure, that all sounds fine. But what matters in terms of gameplay is that while you're on the battlefield this season, you'll always benefit from the Power of the Triune buff. That effect spawns three circles on the map, each of which confers a special benefit while you're standing in it, and each is associated with one of the Prime Evils' nice-guy alter-egos. Run to the Triune of Creation, for instance, and while you're standing in it, any of your skills on cooldown will come off cooldown way faster.
The idea, Blizzard says, is to try to do something unique with this season and encourage players to move around the battlefield more. The Triune buff works on all characters regardless of level, so any player can take advantage of it and enjoy playing a more mobile, aggressive game.
During Season 18, you'll also be able to earn cosmetic rewards that were previously only available during Season 6, which include the chest and gloves items from the Conqueror set.
More information on Season 18, as well as on the latest Diablo 3 patch, can be found on the official site.