Update 5.4 will kick off with a completely new flashpoint called Crisis on Umbara. With three difficulty levels (including master), players will need to bring their A-game (and A-gear) to whip this instance. If players beat the flashpoint a few times, they can gain access to the Umbara stronghold, which is held on a moving train.
The update will also contain more companion customization, the ability to summon companions while moving, higher graphics settings, and the end of Season 8. Discipline balance tuning is scheduled to continue with changes to Lethality, Ruffian, Concealment, Scrapper, Medicine, Sawbones, Fury, Concentration, Carnage, and Combat.
Source: Official forums