Lion's Eye Diamond Old Lion's Eye Diamond was quickly errata'd so that players wouldn't be able to use the mana from the artifact to pay for cards in their hands before the cards were discarded
Combined with Exploration or Fastbond, the Horn quickly becomes an engine from which you can draw three, four, five, or more cards a turn, at no cost
Although your Commander may very well be the most important card in your deck, we would suggest that if you are building your very first list (we built our first when the format was still called Elder Dragon Highlander), that you may be well served in deciding on your individual playstyle and choose your colour(s) first
Impact on the format if banned: While I personally always run 1 in my mono-white sideboards, it's use is limited
Type Line : Legendary Creature - Elemental Couple Message: When this card enters the battlefield, create two 3/3 red Fire creatures
Theres nothing more to say