Complex sentence: When they arrived, Zero had already defeated Belial and blasted him in to a river of molten magma but no sooner had they arrived, the spirits of 100 monsters started to flow towards Belial and formed Beryudora.
Split into simpler sentences: When they arrived, Zero had already defeated Belial and blasted him into a river of molten magma. However no sooner had they arrived, when the spirits of 100 monsters started to flow towards Belial and formed Beryudora.