He can travel to the entity's plane itself.
He needs the "Flower" to get stronger.
What if he is the new killer? What if the cinematic we saw was him becoming one.
He could be a swedish alchemist who tried to do alchemy with plants to try and escape the entities world.
And instead he kept failing, and he lost all hope (reference last entry for: The Alchemist)
And like everyone who lost hope, they became a killer.