Diablo 4's Competitive Tower Gamemode and Leaderboards are going to be set live on January 12th after being delayed for the start of Season 11. The Tower is a competitive endgame activity in which players must complete a randomized, multi-stage, timed dungeon, very similar to The Pit. This is what Blizzard considers the "competitive" activity, where rewards are cosmetic, rather than progression-focused.
The Tower and Leaderboards Guide
The Tower will unlock floors alongside your maximum Pit rank, so no additional grinding is required once the system goes live on Monday. Leaderboards will start in two-week rounds for the rest of the Season, at which point the leaderboards will lock in.