Although he did not compose the selichot that we say today, the approach and style of Rabbi Elazar HaKalir was the basis for most of the selichot.
We do not know when he lived, though likely in the sixth century, and possibly in the second.
His poetic, allegorical style was both praised and criticized.
And he wrote many of the Kinot that we say on Tisha B’Av as well as several of the yotzerot we recite on Rosh Hashana and Yom Kippur, along with the prayers for rain and dew.