The timeline goes something like this.
400 Years Ago Igneel fought with END. This shows he existed back then. Whats not clear is that whether Tartarus existed back then, because the demons of tartarus did not even recognize their master's face. What probably happened was Zeref created them and allowed them to grow, and they had it imprinted inside their heads that END was their master, and that he created tartarus. Their mission was to revive him and "return to Zeref". The facts in their heads were all made up by Zeref. Zeref did not mention about creating tartarus that time.
100 years ago Zeref and Mavis had a conversation where Zeref mentioned that he was SEARCHING for Natsu, thereby implying that Natsu was already taken away by Igneel. This was when Zeref said he created tartarus. Also, Mard Geer is responsible for developing the guild because he has "tartarus" in his name. END as far as the current evidences are considered, has nothing to do with Tartarus at all.