Outer Plane 1

Humanoid Population:

Outer plane 1's invaders were heavily armored individuals in what could be described as medieval-looking mech suits that were outfitted with cannons and stood 12-18ft tall. Their power was quickly found in that a single being could withstand most bullets and warheads. Anti-tank missiles were eventually used to effectiveness against them but often it took multiple or many to take a single individual down. A cannon could also easily take out a building or even a defensive bunker.

Directions in Limbo:

Unknown

Relationship with Earth:

Hostile

Geography

Unknown

Ecosystem

Unknown

Localized Phenomena

Unknown

History

We know that Outer Plane 1 was the first to invade Earth during First Invasion and invaded again during Second Invasion along with Outer Plane 4. Beyond this, there is not a lot of information on the Plane. 
It is unknown during the 1st invasion what the goal or purpose of the invasion was. Guesses on the second invasion are that the invaders wanted individuals for some reason and control over the Flume Crystal.

Type

Plane of Existence