OVA 9: Natsu vs. Mavis Fairy Tail : Season 7 Episode 14

After Erza recruits Natsu to assist Mavis by allowing her to enjoy the qualities of a hot bath, Team Natsu and Juvia take up Warrod's request to look for a hidden treasure buried by the First Master herself 100 years ago, although the task becomes much harder than it was initially let on.