Fusion

Unification (同一化) is an Extra Skill the Goblin Riders and the Star Wolves acquired as a result of Rimuru’s Harvest Festival.

Abilities
The user can physically merge with allies they have a strong relationship with, increasing the unified being's total strength.

In case of the Goblin Riders, they became able to merge into one to move quickly on all fours and get a boost of strength that puts them at about Rank A minus to Rank A. Even the worst among them would dominate the B rank.