Where it fits
The author says Echo Golems collect Soul Blocks in Soul Canyons and carry them toward the surface. They are linked to Singers, which can accept the blocks. This makes the canyon and portal systems feel connected within the mod.
Bonding
Using an Echo Shard can permanently bond an Echo Golem to the player, according to the author. A bonded golem delivers Soul Blocks to the player instead of a Singer. Test in a backup world if the behavior matters to your progression.
Evidence boundary
This is a community mod relationship. Mojang has not confirmed an Echo Golem in the official Sift.