Great Olm

The great olm is the final boss in the Chambers of Xeric raid and is currently the strongest attackable monster in Old School RuneScape, surpassing the Corporeal Beast. It is one of Xeric's prized possessions and is a large white salamander.

Attacks

 * Standard attacks: Olm has two standard attacks; Magic and Ranged. The magic attacks are large green orbs, while the ranged attacks are smaller chunks of crystal. Both deal the same amount of damage.
 * Spheres: Olm randomly picks a sphere (red, green or blue) and sends it at the player. When fired, the player's overhead prayers are dropped, and they will lose a small chunk of prayer points. If not blocked, the player will take heavy damage from it. To block the attack, pray accordingly before the sphere hits you:
 * Red (sphere of aggression): Protect from Melee
 * Green (sphere of accuracy and dexterity): Protect from Missiles
 * Blue (sphere of magical power): Protect from Magic

Throughout the fight, Olm can also use these moves. If the left hand is clenched, these attacks will not occur:
 * Crystal Burst: Olm throws a crystal seedling under every player in the chamber. A few seconds later, the crystals burst and shove the player to a nearby tile if they are on top of them for moderate damage. The left hand will briefly show a crystal icon when using this attack.
 * Lighting: Shoots out lighting running north and/or south of the chamber. Getting hit by a bolt of lighting electrocutes the player, disabling their overhead prayers, dealing moderate damage, and binding them. The left hand will briefly show a lighting bolt when using this attack.
 * Swap: Olm teleports the player(s) for damage. With one person inside the chamber, Olm will instead choose a random tile in the chamber. With two or more, Olm picks those two (or a random two if more) to teleport. To avoid damage, stand on top of the targeted tile/player. The further you are from the target, the more damage is taken. Olm does not use this move on the last phase. The left hand briefly shows a swirl when using this attack.

Olm also has phase-unique attacks:
 * Acid:
 * Acid Spray: Olm spits out several pools of acid around the chamber. Standing on top of an acid pool deals 3-6 damage per tick and inflicts a weak poison effect on the player.
 * Acid Drip: Olm covers a player in acid, causing them to generate acid pools under them. The pools are identically the same as the spray attack.
 * Flame:
 * Deep Burn: Olm shoots out a green fireball at a player, causing them to yell "Burn with me!". The targeted player takes 5 damage every few ticks, dropping stats by 2 each time this occurs for five hits. If it spreads to other players, it will reset the timer and infect uninfected players.
 * Fire Wall: Olm shoots out two walls of fire at a player to trap them. If the player is on a tile that would be occupied by the fire wall, they will leap out and take light damage from it. After the walls are placed, the player(s) trapped inside will take heavy damage if they are not freed. To free the trapped player(s), cast a water spell on a segment of the wall to douse the flame. This can be done outside or inside the wall.
 * Crystal:
 * Falling Crystals: Olm lets out a cry, targeting one player with a red aura. Spiked crystals fall from above, dealing damage to anyone in the targeted tile.
 * Crystal Bombs: Olm shoots out a crystal bomb (shoots more as the fight progresses). The bomb explodes after being deployed for several seconds, covering a 5x5 range. The closer you are to the bomb, the more damage is incurred.

There are also attacks applied during other segments of the fight:
 * Autoheal: During the first three phases, any attacks aimed at the head are completely healed after several ticks have passed. In the third phase, the left arm also gets a variant of this ability where any attacks that deal damage will cause it to heal instead. When this is performed the left hand shows an infinity symbol.
 * Falling Crystals: During phase transitions and the last phase, Olm causes chunks of crystal to fall from above. These crystals have a 3x3 AoE. Getting a direct hit will result in moderate damage, and getting indirectly hit will deal half as much damage. One crystal will always be aimed at a random player in the chamber.
 * Life Siphon: In the last phase, Olm will shoot out two blue projectiles which generate a shield when it hits the ground. Several ticks later, Olm will damage anyone not on the targeted tiles for moderate damage. He will also heal himself for five times the amount of damage he manages to deal to the team.

Fight overview
Olm has at least four phases (more depending on the size of the party), with each successive phase granting Olm extra attacks (he can still use special attacks from old powers at any time as long as they have been accessed). Once you pass the barrier you cannot go back to resupply. You can resupply yourself if you die fighting it.

To progress, disable both arms. They do not have to be disabled at the same time until the penultimate phase. Under no circumstances should you target the head until the start of the penultimate phase, as it has a healing aura that recovers any damage it takes.

On the penultimate phase (3 + 1 more for every 8 players on the team), both arms must be permanently disabled quickly. If one arm is disabled and too much time passes before the other arm is disabled, Olm will regain control of that arm, and it will return with full health. The left arm will start using a rejuvenation aura after the teleport attack has been completed (symbolized as the infinite symbol), during which all damage will heal it instead.

Once both arms are disabled, the final phase begins. Olm's head will remain, and it will attempt to kill the team in a final stand. It will use all attacks from the preceding phases, in addition to crystals falling from the ceiling. Olm also has the ability to siphon the team's hitpoints during this phase. If your team can damage Olm quickly, this attack can be ignored as there will still be crystals falling from above which can deal a lot of damage. Kill the head, and claim your rewards once it dies.

Phase Rotations
Olm's left hand (on the right side when facing Olm) is the hand which should be disabled with melee. This hand is also the hand that indicates which move is coming next out of crystals, lightning wall and teleport attack. At first the next attack may seem random, but each phase does the moves in a set order. This may change between each raid but so far these are the expected rotations.


 * Phase 1: Crystal Burst > Lighting > Swap. After completing an attack rotation, he repeats them again. If the left hand is clenched, none of these special attacks will be performed. The left hand only clenches temporarily, and will automatically let go after a set amount of time has passed or the right hand is disabled. The left hand cannot be attacked while clenched.
 * Phase 2: The same as phase 1.
 * Phase 3: In this phase both hands have to be killed at the same time. After the swap attack will come the autohealing effect for the left hand, after which the attack rotation is repeated again.

Besides the rotations, the head will be attacking. Olm's head will turn to the side with most people on it or the center. When Olm shoots out crystals he is attacking with range, if he shoots out blobs he is attacking with mage. There is no way to predict his attacks, but they will tend to repeat a few times before switching.

Trivia

 * An is a species of aquatic serpentine salamanders living in lakes deep underground.
 * Olm made a teaser appearance on the 2016 Christmas event, where it killed a worker named Ulina in a future vision.
 * Great Olm is the first attackable NPC with a combat level above 1000.
 * The great olm is the only monster in the Chambers of Xeric that has a defined combat level; all other monsters encountered inside do not have any known combat level.
 * In addition, it's arms also have a defined combat level (though they are still part of it, so it is still technically one monster). Interestingly enough, the left arm boasts a higher combat level than the right arm.