Horus held back against the Emperor the whole time in their fight

You draw the warp into you so that He can see it. You need to make Him understand, for He does not seem to have grasped it yet, despite that acclaimed wisdom He boasts of. Your power is infinite. His is not. No matter how many times He gets back up, to renew the fight for another desperate go-around, He is merely postponing the inevitable. He is a warp-attuned creature of great power, by any mortal standards, and rightly has been feared His entire life. But His great strength is finite. You are an infinite being of the infinite warp. Your power will never run out, and can never be sapped, no matter how many or how grievous the injuries He inflicts upon you. You cannot be killed.
This contest was over before it began. It was superfluous. You only permitted the fight to take place at all because He seemed to need it. It was all for show, a demonstration of your new-found state, a symbolic, ritual act to consecrate your reign. You fight only to wear Him down to nothing until He is rendered entirely helpless and subject to your will.
Surely He sees that now?

-Siege of Terra: The End And The Death - Volume three