Art by Feylen
At the end of Episode 54, Umbrasyl took off near the end of the fourth round. Assuming initiative order holds steady, Pike, Vax, and Scanlan will get the next turns to finish the round, respectively.
The since-abandoned Vox Machina wiki, originally maintained by Matthew Mercer himself, says this about Gatshadow:
The ominous mountain that lords over Westruun and the southern base of the Cliffkeep Mountains, Gatshadow's peak is nearly always obscured by storm clouds or mist. The actual stone that comprises the monstrous landmass is of a darker, more dense stone that is considered to be the finest for crafting on this side of the world (hence why many craftsmen and smiths make the dangerous journey there to harvest the rock... and not all return.
Several critters have questioned whether or not Umbrasyl could have set up his lair in Gatshadow in the time since the Conclave announced his presence, as black dragons tend to lair in swamps rather than mountains. However, we consider the perpetually-obscured peak of the mountain as a potential clue that Umbrasyl may have set up shop there long before.
With that in mind, should the battle continue in Umbrasyl’s lair, that will add a number of wrinkles to the battle. The first will be traversing the dangerous journey to the lair itself, which will magically take twice as long to traverse as a regional effect, and will be guarded with traps as is the way of paranoid dragons. The second will be lair actions, an extra ability that Umbrasyl will be able to evoke near the beginning of each round that will greatly increase the difficulty of the battle.
The third wrinkle (which is entirely speculation on Lauren’s and Andrew’s parts) is the potential communication he will make with the rest of the Conclave. Brimscythe had a series of gems that granted him contact with the rest of the dragons, and there is reason to believe that Umbrasyl will have the same system in place in his residence. While it is unlikely that the Conclave would arrive in time to provide support, paranoia and defenses will likely be increased, and any hints of resistance will be immediately crushed. Westruun may also not be as safe to return to as Vox Machina might have hoped.
But, hey, we can speculate, or we can just wait and see what happens. #IsItTwoThursdaysFromNowYet