Monsters are neutral units in League of Legends. Unlike minions, monsters do not fight for either team, and will only do so if provoked (the exception of this being the Rift Scuttler, which simply runs away when attacked).
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 mutated the local wildlife. From giant frogs, Giant Birds to Two-headed Wolves it has also attracted the attention of powerful Dragons who currently reside 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 Rift's 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 Black Mist. Numerous individuals and creatures now serve the Harrowing's will.
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, their "patience" bar depletes. This bar depletes faster the farther you are from the monster's spawn spot and the less you attack them after having provoked them. Non-epic monsters' patience will also deplete once they change target. They can change it up to 6 times after which the patience bar reaches 0.
Once the patience bar is depleted, the monster will head back to their spawn point, as well as massively regenerating their health over the duration. Even when attacked during this retreat, the monster will not be provoked until they reset upon reaching spawn.
Once monsters get provoked they will detect champions even if these are hidden in the fog of war, going towards the closest champion 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 Rift Scuttler is the exception to this, as it does not attack even when provoked.
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 champions (the designated jungler, plus one or more allied champion/s) attacking a monster camp together, clearing it faster than if done solo.
Typically, the non-jungler will perform as the "meatshield," taking damage instead of the jungler. This allows the jungler to be able to clear the next camps safely.
The non-jungler is also tasked with not dealing the last hit on the monster, stopping when the monster is low on health. Dealing this final blow can be fatal to the jungler's leveling and gold income, as the experience and gold is given to the non-jungler, not the jungler.
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. A camp's respawn timer can be known by pressing TAB. However, there is a prerequisite of having you or an allied champion witness the camp when it was cleared, or else the respawn timer remains unknown. This prevents the opposing team from knowing when a camp has been cleared and when it will respawn.
A monster camp will not respawn until all of the monsters in the camp have been slain. For example, if the Greater Murk Wolf has been slain, but at least one of its two Murk Wolf companions has not, the camp's respawn timer will not start until the last Wolf has been slain. Those invading the opposing team's jungle may deliberately do this in an effort to slow the opposing jungler's gold/experience income.
This tactic can be counter-productive if invasions into the enemy jungle is too frequent, or if the enemy jungler kills the leftover monster/s too soon that the purpose of delaying the camps respawn is minimal.
As a general rule, killing a monster rewards its killer with gold and experience points.
Additionally, certain large monsters offer greater rewards when killed, such as an individual buff, a team buff, or a global gold reward.
The Crest of Insight and Crest of Cinders buffs are gained respectively from the Blue Sentinel and the Red Brambleback, and may be transferred to another champion. This occurs when a champion kills someone holding a crest, causing the crest to be transferred to the killer and resetting the buffs duration. Either buff disappears entirely if the champion holding the crest is executed by a minion, monster, or tower, or when the crest simply wear offs on its own after a few minutes.
The Eye of the Herald buff can be picked up by the team that delivered the killing blow to the Rift Herald, lasting for 4 minutes. This buff allows the holder to summon the Rift Herald on their team, which will then head to the closest lane to assist in pushing the lane until it dies.
The stacking Dragon Slayer buffs acquired for slaying the Dragon are applied to all team members, whether living or dead, and last indefinitely. The exception to this is the Aspect of the Dragon from the Elder Dragon, which is only granted to living teammates and lasts for a few minutes and is lost upon death.
The Hand of Baron buff is only granted to living teammates when Baron Nashor is killed. The buff lasts for a few minutes and is lost upon death.
In addition to the above buffs, which are acquired by killing their respective monsters, large monsters heal any champion who casts the Smite spell on them for 70 (+ 10% maximum health).
Jungle monsters now have a Patience characteristic. Monsters no longer reset their aggression based on distance or positioning - Instead, once the monster's Patience is reduced to zero, the monster will reset and return to their original position. During this reset, they rapidly regenerate health and do not respond to further aggression.
Monsters lose Patience under the following conditions:
When a monster is in an aggressive state but can't find any targets to attack (ex. its target died or became untargetable)
When a monster's target is too far away from the monster's initial spawn location or the monster is VERY far away from its initial spawn location
When the monster is forced to switch targets
Large monsters display their Patience in a meter above their heads.
Once a monster has recovered to 90% health, they regain all of their Patience.
Abilities that pull or displace monsters no longer completely reset the monster's aggro if they already had a previous target.
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.