Intimacy
That scene is an extension from the episode in season 4 where Quintin and Elliot live 50 years together to find the true meaning of life so the key will reveal itself.
Once they get the key, they teleport back to the throne room which is where Elliot faced his greatest fear. Quintin reveals his love for Elliot and suggests they actually have a real relationship together. Elliot has had friends, but always kept them at arms length. He never had a real, serious, intimate relationship with anyone and the idea terrified him.