City of light Erabacus

There exists a hidden city of bipedal animals and other races. The city is the center of learning for many as it exists at all points and all times. Getting there one must the right harmonic key or be a citizen of that place. Just I've been there before. They've tried trust humans before but many have broken one of the cardinal rules about exposing the city. There minds were erased. Its where I learned many of the advanced magics and was the hub that I used when I went on missions for great mother. I hold citizenship there and to home. They remain hidden to outsiders. It was during my time away from home before I returned there. I think there were other ponies there too. The cities name was Erabacus the city of light. My mother's sister the administrator of the city.