After "The Ending of the End", the real Grogar threatens Equestria. At first was almost succefull conquest, but defeated by Mane 6, he then black out. Later wake up in unfamiliar place with Tirek ( restore his powers and magic ), Sombra and Storm King ( he gain control of storm and lightnings ). Before they began bickering, they hit by strange magic and began fuse. The villians are bewilder what happened them and turn into one powerfull being, that will call Alastor.

Here is art fusion of villians
https://www.deviantart.com/zetikoopa/art/Villians-fusion-940670991