Summon Zerg from Another World

You can only summon one contracted beast? Sorry, I can summon a legion, and it is the most ferocious Zerg in StarCraft! Your guardian spirit can never grow? Sorry, my guardian soul can grow infinitely, and it can change into whatever it wants! Do you have a unique skill for your whole life? Why didn't you tell me earlier? The largest secret book wholesaler in the continent welcomes you! PS: Although the introduction is very YD, this article is a growth type, a slow-burning type, and it is definitely not YY-level!