The Creation Circle is used to create 5-star and 6-star heroes by fusing weaker heroes together. All 3-star heroes are used in a 5-star hero fusing. Only the level of the first hero is important as the fused hero will come out with the same level and equipment as it. Any promotion stones and spirit used to upgrade the other heroes will be returned, although the gold spent will be lost.
To fuse 5 star heroes, you will need two of the 4 star version of the hero, four 3 star heroes of the same faction, and six 4 star heroes from the same faction.
For 6 star heroes, you will need two of the 5 star version of the hero, and four 5 star heroes from the same faction.
All 4 star heroes can be fused into 5 star versions, all heroes that have a 5 star version but not a 4 star version can be fused into 6 star heroes, but only some heroes that have a 4 star version can be fused into 6 star versions.