Crystal, The Giantess!

This idea was for a fantasy world but doesn't have to be. The main character stumbles on Crystal, a massive futanari/woman giantess who is pent-up and horny. She immediately takes control of the main character(s) using her size and begins to use them however she wants, likely as a sex toy in some way. She really enjoys cock vore with her hyper-sized dick. It is possible characters may become lost in her balls for a while. (If you decide you want the futa version of Crystal.)