Found on pixiv:
http://www.pixiv.net/member_illust.p...st_id=44972753
Google translate (and a little modifying on my part) has this to say about the description:
"if yuna and tidus never met..
during the ultimate summon、yuna was affected by the impact and became the Sin's summoner
yuna is attuned to Sin's desires、becoming the embodiment of slaughter and destruction
yuna is under the full control of sin, and her new mission was clear: the destruction of Spira"
(translation may or may not be completely off)