During a press conference that announced “The Vana’diel Project” on March 18th, 2015, Square Enix announced a mobile game set in the world of Vana’diel. Final Fantasy Grandmasters would be a new Final Fantasy XI related title, but also one of two mobile titles announced at the event. The other was a mobile version of Final Fantasy XI that would be developed in partnership with Nexon (the project was confirmed to have been cancelled in February, 2021)
Final Fantasy Grandmasters launched on October 1, 2015 and eventually ended service on April 25, 2019. It was only released in Japan.
Sources: https://press.na.square-enix.com/FINAL-FANTASY-XI-Seekers-of-Adoulin, https://youtu.be/I9dkdRvvp2s?si=RQOm8u_Ig4CKKzLv
FFMMO Archive 