Since you killed me, I am no longer bound to the Horn of Valere. A close friend of mine finds it and blows it, intending to avenge me. It just so happens that, when I died, I was made a Hero of the Horn, and am now summoned from beyond the grave with all the other Heroes. We ride to the hill, and take it back with the force of a legend.
(In case anyone who is reading the Wheel of Time thinks that this is referencing a spoiler, dont' worry, it's not; I'm just bullshitting for the hill).