Altars are a new system in History of Middle-Earth.
You craft them through the forge system!
What do they do? there are 2 things. Just placing a altar with the special block under it. Will make your faction invisible in the biome.
You can only do this if you own the full territory!
You can also heal or corrupt the biome. this has 2 tiers. a bit healed/corrupted or fully healed/corrupted.
You do this by placing the special altar corner blocks & special blocks in specific places around the altar.
The special blocks are Block of Edhelvir to heal it and Block of Gulduril to corrupt it.
NOTE: Only one ALTAR per biome is possibile!
NOTE: Altars WILL explode even when bannered protected on placing the altar.
NOTE: Altars are BIOME bound. so not territory bound.
To just make the hidding in a biome. do the following thing:

TO make it tier one with the blocks (no corners yet)

This is how the altar needs to be to recolor the biome a bit.

Fully recolor the biome with this!

Only those blocks and exact placement are needed!

Without beam to count better :)

Wool to showcase it in terms of distance