I have to agree with NB on this point. Though I hate x-listing when it gets out of hand, I hate errata even more. It is hard enough to remember now what cards got errata and what the errata is unless you bring the print out with you everytime you play a game. X-listing a card in one format is fine, since you can just avoid a format when playing. Giving a card errata effects all formats the game is played in (including open and casual). Open and casual games are ment to test you in every aspect of the game. Since the game is dead anyway and no new sets will be coming out I say (if we do anything at all) just x-list
New Chapter in standard. The Horn deck will get smashed in Open format anyway. Keeping it legal in expanded would even be fine as there are so many crazy combos in that format too.
Like many have stated, the Horn deck can be dominated easily with mass archery, good corruption and other decks that aren't just focused on a horn kill

. You can even set up a good swarm deck and play (or swap the site to)
Cavern Entrance and that can even beat the horn. Granted the horn deck helps your shadow out but that is only half the fight. Freeps can get beat, and just because your deck is filtered to draw shadow every time doesn't mean your shadow will defeat your opponents fellowship.
Giving errata to a card not only causes mass confusion but ruins the game in all formats not just the format it causes problems in

.
Thats just my


, not that it will matter much

. Anyway forgive any errors in grammar or spelling, I rarely proof read my longer posts for errors.