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Challenge Watchers belong in the Utility Abnormal tier, as they are a utility mob, not an Organoid.
Spawning
Spawned when a Brain Remnant block is lit on fire, in which multiple tentacles will come out of it and flail around, playing a screaming sound before exploding and spawning the Challenge Watcher.
Unique Behavior
The Challenge Watcher is immune to all damage types except for void damage.
As soon as a Challenge Watcher spawns, it will begin a raid sequence. These raids scale in power and length depending on the power of each target within 16 blocks. There is no max to the number of waves or levels it can spawn, but waves beyond the 3rd level will just use the level 3 stats.
Wave scaling:
Armor: For every 5 armor points, it increases the size of the wave by 1. For every 7 armor points, it increases the level of the wave by 1.
Health: For every 4 health points, it increases the size of the wave by 1. For every 50 health points, it increases the level of the wave by 1.
Special Spawns: If the target has the Symbiosis effect, it'll increase the number of special spawns by 1. If the target can hold items, it'll look for edible and enchanted items. For every 32 edible items, it'll increase the number of special spawns by 1. For every 3 instances of a Reagent enchantment, it'll increase the number of special spawns by 1.
Wave 1 and higher will spawn Verwahrungs within 16 blocks of the Challenge Watcher. Verwahrungs spawned by Challenge Watchers don't contain the normal list of mobs that default Verwahrungs have and use a special list depending on the wave level.
Wave 1 spawns: Infected Human, Infected Husk, Infected Drowned, Infected Villager, Infected Pillager, Infected Witch, and Infected Adventurer.
Wave 2 spawns: Infected Human, Infected Husk, Infected Villager, Infected Pillager, Infected Witch, Infected Adventurer, Knight, Braiomil, Jagdhund, Slasher, Howler, Brute, and Nuckelavee.
Wave 3 and above spawns: Knight, Griefer, Braiomil, Phayres, Verdathorn, Jagdhund, Scavenger, Bloater, Leaper, Slasher, Spitter, Volatile, Mephitic, Howler, Stalker, Brute, Nuckelavee, Infected Vindicator, and Infected Evoker.
Wave 2 and higher can spawn Howitzer bombs. Every time a Verwahrung is spawned, there is a base 5% chance a Howitzer bomb will spawn within 32 blocks of the Challenge Watcher (ignoring Y levels.) Each wave level beyond 2 will increase the chance for a Howitzer bomb to spawn by 5%.
Special spawns are unique mobs that have a 10% chance to replace normal mobs in Verwahrungs within 16 blocks of the Challenge Watcher if the special spawn count is above 0. Special mobs that can spawn include Inquisitors, Brotkatzes, Groberfubs, Wendigos, Ogres, Axtwerfers, Hexenmeisters, Plagued, Lacerators, Bioblobs, Sauglings, Gastgebers, Specters, and Infested Constructs. Each wave can have a max of 3 special spawns.
If the Challenge Watcher detects a target that is at least 3 blocks above it, it will spawn multiple Usurpers to stop players from cheesing the fight. It will check for mobs above it every 2 seconds.
If the Challenge Watcher cannot spawn any more mobs and there are less than 4 spore mobs within an 8-block radius, the Challenge Watcher will burrow, leaving behind rewards. Alternatively, Challenge Watchers can be right clicked with a The Eye of the Vigil items to despawn it, leaving no rewards.
The amount and type of items it drops depend on how strong the raid was. The following are the items that may drop: Fleshy Bone, Hardened Bind, Fleshy Claw, Living Core, Spine, Armor Plate, Dense Muscle, Altered Spleen, Corrosive Sack, Sickle Fragment, The Eye of the Vigil, Symbiotic Reagent, Cryogenic Reagent, Gastric Reagent, Corrosive Reagent, Serrated Reagent, Voracious Reagent, Calcified Tumor, Frozen Tumor, Sicken Tumor, Bile Tumor, Flesh Horse Armor, Plated Horse Armor, and Living Horse Armor.
If the player leaves the 32 block wide arena (16 blocks on either side of the Challenge Watcher), they won't be calculated for the wave size of the next round. Additionally, if they are not present in the arena for the final wave, rewards will not be dropped for that player.
Behavior
Challenge Watchers are incapable of movement.
Able to breath underwater.
Challenge Watcher
Statistics
HP: 1 (x0.5 hearts)
Armor: 1 (x0.5 bars)
Damage: 0 (x0 hearts)
ID: spore:arena_tendril
Threat Level: Unknown
Trivia
Challenge Watchers were originally named 'Arena Tendrils'. It's unknown why the name was changed.
The Challenge Watcher is able to take void damage because during beta testing, Dr. Pilot (a staff member) attempted to teleport all the Challenge Watchers in the world to the void in order to clear them, but instead it ended up lagging her game, so they were made to be able to take void damage.
The raid ability of the Challenge Watcher is very similar to the Scent System from Scape and Run: Parasites. This was unintentional, as the mod creator only found out about it after someone pointed it out.
The reason the Challenge Watcher is able to summon Howitzer bombs is because it is a collab with a nearby Howitzer and Delusioner, according to Harby.
Gallery
A picture posted in the Discord server teasing the Challenge Watcher before it was released.