As Viessa stated, the Eternal Oasis was the heart of the Elven Race. It was where the Great Elven Clan was located as well as the Emplenan Abode.
If the Elven Race were to face invasion, the Eternal Oasis served as their final sanctuary where their most powerful tools of defense reside.
Thus, although Venali hated the fact a human was being introduced to this place, he understood a sole human could cause no trouble within that space.
Meanwhile, upon laying eyes upon the scene before his eyes, Kieran stood with his mouth agape. 'I've yet to enter the Eternal Oasis, but this energy I'm feeling… it's remarkable.'
Out of pure instinct, Kieran extended his hand to pass through the threshold of the gateway Viessa prepared. Due to the strange laws that governed the Eternal Oasis, upon drawing near the gateway, Kieran realized that everything beyond the threshold appeared inverted.