The Beachhead map is a unique Harbinger Map that takes you to an area called The Beachhead. This area will have lots of Harbingers, and is considered the main boss area of the Harbinger League.
There are a total of 3 "different" Beachhead maps, one for each map color. The White Beachhead Map is tier 5, the Yellow is tier 10, and the Red Beachhead Map is tier 15.
In order to get a Beachhead map you must upgrade maps using the Harbinger's Orb. You can get this by trading or collecting Harbinger's Shard from killing Harbingers in the game. Each time you upgrade a map it has a 20 % chance to turn into a Beachhead Map of the same color. This means that tier 1-5 will be made into White maps, tier 6-10 into Yellow and Tier 11+ into Red Beachhead Maps.