The second Mahou Shoujo Taisen illustration contest by 2.5 Dimension TV on Pixiv.
It ran from 2013-11-28 to 2014-1-29 and there were about 1300 entries.
Contest terms
Contest results
Grand Prize (The terms said at least 1, but they actually awarded 3) - Prize: 50,000 yen and the appearance of the character in the Mahou Shoujo Taisen anime