The Eastern Strait is a location first, and only seen in How to Train Your Dragon: The Hidden World.
Not much is known about this location. According to a map shown in the movie, it is a wide strait between the Mainland and another large island. Judging by the name, it is east of the Warlords' Headquarters.
|“||The Dragon Riders attacked us here, off the Eastern Strait. [src]||”|
|— Ivar the Witless|
The Eastern Strait appears very briefly in the movie, serving as the outpost of Ivar's Trapper Barge. Due to the location being shrouded in thick fog, very little of the surrounding areas could be seen.