Baron Nashor

Baron Nashor is the most powerful neutral monster on Summoner's Rift in League of Legends.

It is described in-game as follows: "This is a very powerful hostile monster, approach with caution! Killing this monster will yield great monetary and magical rewards to the team, but it should not be confronted alone".

Killing the Baron gives the buff to all living champions on the team, which lasts 4 minutes total (Runic Affinity doesn't work on this buff). This buff gives massively increased attack damage and ability power as well as a bonus to health and mana regeneration. When Nashor is killed, each player on the team is also granted the following:
 * 900 Experience Points
 * 300 Gold

Lore
The Journal of Justice clarifies this about Baron Nashor :

''The many neutral monsters on the Fields of Justice are constructs inspired by real creatures from Runeterra. The Baron Nashor you see on the map is actually an incomplete facsimile of the true Baron Nashor, who is now the subject of myth and legend. The story goes that Baron Nashor was a ferocious worm that ruled the Serpentine River, which flows through Summoner’s Rift. He was such a fearsome and powerful beast that even the armies of Demacia and Noxus steered clear of the river, unwilling to chance an encounter with the Baron.''

''When Summoner’s Rift was constructed, the Baron vanished, leaving no conclusive evidence as to where he went or why he left in the first place. Obviously the rumor-mill has churned out many explanations, but all we know for sure is that the Senior Arcane Architect for the Summoner’s Rift arena was said to have crafted the version you see today from no more than a molted scale and interviews with those who had seen the beast and lived.''

Abilities
Baron Nashor has four abilities. While they are not revealed in-game on a death screen, their debuffs and animations are visible and distinctive.

Baron Nashor will usually start with Wrath of the Ancients. He will continue using Wrath of the Ancients unless he feels you are doing too much physical damage, at which point he will start using Voracious Corrosion to keep its debuff on you while alternating with Wrath for damage. He will mix in the knockback and knockup moderately often throughout the fight.

Additionally, Baron Nashor has the passive ability to knock back any wards and being placed too close to its body for a short range. Noxious Traps are pushed far enough away so that their splash damage does not reach Baron Nashor. Wards are pushed away so that they are not placed inside Baron Nashor; otherwise, the ward would be untargetable and unable to be destroyed without first killing Baron Nashor.

Gameplay

 * Due to the debuffs that the Baron can apply to you, it is highly recomended that you have full vision of the enemy team, as the stacking debuff applied (25% x 10 times = 250% Damage Increase) renders your team EXTREMELY vulnerable to AOE magic damage casters since the increase in magic damage devastates your team from any source of magic damage (including most global ultimates).
 * Baron Nashor is often a key objective in competitive games because of its massive gold bonus (+300g/player) and global buff.
 * The killer receives an additional 25 bonus gold.
 * Baron Nashor is immune to all forms of Crowd Control. Killing him should not be attempted until mid or late game.
 * Due to its power you should not attempt to solo it, this feat is usually not possible unless you have a full item build with a lot of survivability.
 * Killing Baron with a team is fairly simple. Baron primarily attacks the target closest to him so you can distribute damage by positioning characters closer or farther away from him.
 * The enemy team likely will have a ward placed to see when yours initiates on Baron. If you do not destroy these wards with the use of a or, they may be able to assault your team while you are weakened by Baron and subsequently steal the  buff/kill several players, putting you at a disadvantage.
 * Baron Nashor's buff is given to the team that deals the killing blow on Baron. This mechanic means that even if one team is incapable of directly fighting the enemy team, a jungler with the Smite summoner spell can possibly swoop in and steal the Baron with a well timed . This is almost definitely a suicide mission and is an extremely high risk maneuver. However, successfully stealing Baron can turn the tide of a game dramatically. Being able to steal or prevent the stealing of Baron (or other key monsters) is also the main argument for junglers to always have Smite, even if they can deal with monster camps without it.


 * In the same vein, Champions possessing abilities with a considerable range can attempt to do the same with less risk by dealing the killing blow from a distance. However, this is difficult to attempt because perfect timing and clear vision of the battle are required to pull it off. Some Champions that can do this are:, , , , , , , , , ,.
 * When Baron dies, the buff is only granted to those team members who are currently alive. Dead team members will NOT respawn with the buff. Furthermore, this buff is the only buff in Summoner's Rift that does not transfer via kills. If a champion is killed with this buff, the buff is destroyed and cannot be taken again until Baron Nashor respawns.

Trivia

 * During the 2010 Snowdown Showdown, Baron Nashor wore three santa hats, one for each of his three faces.
 * Also, during the 2012 Snowdown Showdown, players can each earn a Summoner Icon featuring Baron Nashor's main head with a santa hat on.
 * His name is a tribute to Roshan, (his name being an anagram of Roshan) a monster filling a similar role in Defense of the Ancients.
 * A box labeled "Baron Acid" can be seen in the preview video for the launch of the League of Legends' Mac Version.
 * CEO and Co-Founder, Brandon 'Ryze' Beck has stated on Twitter that they are currently planning to give Baron Nashor a visual upgrade, with his latest concept revealed in the second League of Legends cinematic, A Twist of Fate.
 * In the death recap screen, Voracious Corrosion is labeled as "WormAttack" and Wrath of the Ancients "wrathdamage".