A Monster is a classification for neutral units in League of Legends. Monsters do not fight for either team, unlike minions, and will not automatically attack a champion unless they are attacked (with the exception of the Rift Scuttler, which simply runs away).
Historically, the forest where the Rift resides was the site of several battles between the Protectorate (Stag crest) and Magelords (Owl crest) summoners, two sides who sought out to use the natural magical energy of the area for their personal use.
The large amount of concentrated magic has it mutated the local wildlife. From giant frogs, giant birds to two headed wolves it has also attracted the attention of a powerful Dragon who currently resides in one of the battle torn ruins. Sentinels, Bramblebacks and Krugs are creatures made from local flora and earth infused to construct a magical being. The Rifts unstable magic has inadvertently cause the reemergence of Baron Nashor, a large Serpent/Worm largely believed to be dead for centuries. It has his corruption affecting the land around him.
After the fall of the Shadow Isles, the islands inhabitants were changed by the Myst. Numerous individuals and creatures now serve the Harrowings will.
The Vilemaw is an ancient spider of unknown origin currently residing in the cursed forest. The "Cult of the Spider" has most of its followers located in Noxus and Demacia. Elise serves as its High Priestess, traveling to and from between the two nations and the Shadow Isles, bringing her followers to live with the Spider God, Vilemaw, for eternity, or so they believe.
Monsters reside in the jungle between the lanes, and will not move from their designated camp until attacked. When attacked, the monster will move towards and attack the nearest champion who recently dealt damage to it, up to a certain distance from its spawning point (its "leash" distance). If the monster is unable to attack any target or reaches its leash range, it will walk back to its original spawn point, regenerating to its full health within a few seconds. If attacked on its way back to its spawn point, this health regeneration will cease and it will resume trying to attack the champion who most recently dealt damage to it. Since monsters attack the nearest champion, they can select a new target after having attacked. There is an internal counter for these target changes, and after having changed target 6 times, a monster will return to its spawn point and fully regenerate even if attacked.
Monsters do not immediately "forget" champions that hide in bushes, and try to walk into the bush to attack champions who recently damaged them, up to their leash range (Leash range: 700).
A monster that can see a champion provides vision of this champion to all of the monsters in its camp.
The practice of "Leashing" monsters when they spawn for the first time at the start of the game is an important component of Jungling. Leashing involves two or more allied champions (one of them being the jungler) attacking the same monster camp at the same time. When the monster's health is low, The non-jungler champions will cease their attacks and head to their respective lanes, allowing the jungler to last hit the monsters for the experience and gold reward. If done correctly, this allows the jungler to take less damage from the first monster camp he fights thus making his first jungle run quicker and safer, and allowing him to have the maximum amount of health if the jungler goes to gank a lane afterwards.
After a camp has been completely cleared, it will remain empty for an amount of time specific to that camp (its "respawn time"), then all of the monsters will reappear. If at least one allied champion is able to witness the death of an entire monster camp that provides buffs, either by being present for it or by seeing it with a ward, then they will have knowledge of that camp's respawn timer, visible by pressing the TAB key.
A monster camp will not respawn unless all of its creeps have been slain. For example, if the Red Brambleback has been slain, but at least one of its two Cinderling companions has not, the red brambleback respawn timer will not start until the last cinderling has been slain. Jungle invaders may deliberately do this to slow the opposing jungler's gold/experience income.
This tactic can be counter-productive if :
The enemy team consistently, immediately kills the leftover monsters: in that case, not killing the last small monster is just a plain loss of gold/xp.
You plan on regularly invading the enemy jungle, and expect to steal the enemy camp again when it respawns.
As a general rule, killing a monster rewards its killer with gold and experience points. The experience is split between allied champion within a 400-unit range, while the gold goes to the last-hitting champion. And like minions, the gold and experience rewards from slaying monsters increases as the game progresses.
Additionally, certain large monsters offer greater rewards when killed, like an individual buff, a team buff, or a gold global reward. Some rewards go to the entire team, some to all of its living members.
The stacking Dragon Slayer buffs acquired for slaying the Dragon are applied to all team members, whether living or dead, and last indefinitely.
The Hand of Baron buff is only applied to living teammates when Baron Nashor is killed. The buff lasts for three minutes and is lost upon death.
The Crest of Insight and Crest of Cinders buffs respectively are transferred to the champion who kills the champion currently possessing them, resetting their duration in the process. These two buffs disappear entirely if the champion holding them is executed by a minion, monster or tower, and wear off after 2 minutes.
In addition to the above buffs, which are acquired by killing their respective monsters, certain monsters provide temporary buffs to any champion who casts the Smite spell on them.
Using Smite on the Greater Murk Wolf will summon a nature spirit that grants sight over the nearby crossroads for 95 seconds. Whenever an enemy champion comes in range of the nature spirit, it will chase them for a short duration. This bonus can only be claimed once per spawn.
Using Smite upon the Crimson Raptor will grant Razor Sharp for 90 seconds, which will grant you true sight for 10 seconds the next time you are spotted by an enemy ward. This bonus can only be claimed once per spawn.
Using Smite upon Ancient Krug will grant you Gift of Heavy Hands for 90 seconds, causing every 6th basic attack to stun minions and monsters. Additionally, you can attack a tower to expend the buff early - dealing a large amount of bonus true damage (65 + 15 x level). This bonus can only be claimed once per spawn, and only by smiting the Ancient Krug - Smite cannot be used on the small Krug.
Using Smite upon Gromp will grant Gift of the Toadstool for 90 seconds, which poisons any units (including champions) that attack you: dealing 10 + (5% of bonus health) magic damage over 3 seconds. This bonus can only be claimed once per spawn.
Using Smite upon the Blue Sentinel will restore a large amount of mana over 2 seconds (25% of your maximum mana). This bonus can only be claimed once per spawn.
Using Smite on Red Brambleback will restore a large amount of health instantly (20% of your maximum health). This bonus can only be claimed once per spawn.
List of Monsters and Rewards
This table lists the initial statistics each monster grants, and the value of gold and experience granted when killing it.
The monsters spawn level 2 (6 for the Dragon) in Summoner's Rift.
The monsters spawn level 2 in The Twisted Treeline.
Over time the monsters will grant additional rewards; monsters will level up after having been killed, except Dragon, Baron, and Vilemaw, who will level up throughout the game. Check the individual monster pages to see these level up values.
Upon killing Vilemaw, champions become ghosted, ignoring all unit collision for the duration of the buff. Additionally, nearby minions gain aura bonuses, and briefly terrorize nearby enemy minions on first contact.