Princess Hisui reveals her "Eclipse 2" plan, explaining that the outcome of the Grand Magic Games is the key to preventing the dragon invasion. Technical Guide for this File Episode Number: Season 2, Episode 11 (Overall Episode 186). Language: SUB ITA (Japanese audio with Italian subtitles).
If you are actually looking for the latest sequel series, Fairy Tail: 100 Years Quest , Episode 11 of that series focuses on Natsu's encounter with the spirit of Zeref and Mavis in a subconscious "Beta Heaven". However, based on the standard naming for the 2014 series, the guide above for Episode 186 is most likely what you need.
The battle between the Iron Dragon Slayer and the Shadow Dragon Slayer begins in the streets of Crocus.