From Vavacung's Second World collection, Volume 5. https://300351.e-junkie.com/product/1525463#Second+World+Vol.+5

King Sombra survives the blast from Crystal Heart and recreates himself from the horn alone. But suddenly, someone summons him from Equestria to another world. In this world, he found a strange deer. Eager to test the strength of his new body, he jumps into a fight with this deer... Without knowing how cold this deer can be...