There, done. We now can have renewable guardian horns.
For those who don't know, the Shadow Atrium (Shadow Heart) is dropped after killing a level 3 shadow piece, which is quite difficult considering the work the needs to be done to even attempt a fight. Right now, the heart has no use, so I thought that using it to revive the Ancient Guardian would be a implementable idea. A pedestal should be placed in the middle of where the ancient guardian dwells and all you need to do is interact with it using a Shadow Atrium.