You can use Leagues for anything you like -- costume contests, SG events, community events, RPing, etc. But if you're asking if there's actual game mission content that requires/utilizes Leagues other than iTrails -- at the current time, no, not as such.
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StainedGlassScarlet - L50 Spines/Inv Scrapper | Badges: 1,396
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Events that require League:
- All the Incarnate Trials (BAF, Lambda, Keyes, UG, and the two on Beta: TPN and MoM).
- Death From Below sewers trial
- Halloween Haunted House Trial
- Lord of Winter's Realm (last year's multi-team event which will now be through the LFG Team up Teleporter)
Multi-Team Events that benefit from the League-ing the teams:
- The Cathedral of Pain Trial
- Hami Raids
- MotherShip Raids
- Deadly Apocalypse (the Halloween 'banners' event)
- Zone Invasions: zombies, riktis, Giant Monster hunts, and whatever Dev/CommunityMod run events
And any large-group social event, like costume contests for an easy way to see who's involved and a make-shift communications channel.
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What content is available that will let you utilize Leagues (i.e. 9 or more players)? Is there anything besides the trials? Anything other than level 50 content?