The Thorium Mod expands on the number of bosses that can be challenged by players at various points in the game. With exception to a new final boss, each of the bosses added in the mod are strictly optional and cannot spawn naturally without the use of their respective boss summoning items (or alternative methods) to initiate the fight. However, defeating these bosses may be beneficial to players, as their drops supply additional powerful options to assist players with progressing further into the game. Additionally, they grant access to new Town NPCs who can provide helpful items and other services.
The mod additionally adds mini-bosses, special enemies that can only be fought only once per world, and will not respawn after their defeat. They are comparatively stronger than standard enemies, but overall provide less of a challenge than proper bosses.
Similar to vanilla Terraria, players that are underprepared may find certain bosses challenging, and players that are over-prepared may find other boss fights too easy. As such, it is up to the player's discretion to adjust the angle at which they tackle the different bosses, ensuring they are able to enjoy fighting them to the best degree.
- Summoned with a
Music: Grand Thunder Bird
The Grand Thunder Bird is a tan and turquoise feathered bird with 1,500 / 2,000 health. It is summoned with a Grand Flare Gun using Storm Flares during the Day in the Desert. It has a delayed charge attack that can be halted with a melee strike, and will also summon Storm Hatchlings, create storm clouds over the player's head that rain down tornado-inducing lightning bolts, and utter loud cries that bring down bolts of energy from the sky. It is considered the easiest boss in Terraria, but may still prove a challenge to unprepared players. Defeating The Grand Thunder Bird is required for the Desert Acolyte NPC to move in.
The Grand Thunder Bird drops lightning bird-themed weapons and accessories, as well as Sandstone Ingots, an early game crafting material. It also drops the Zephyr's Feather in Expert Mode, an accessory, which allows limited ascending and gliding with the help of a Grand Thunder Bird hatchling.
- Summoned with a
Music: In Over Your Head
The Queen Jellyfish is a large pink jellyfish that has the Diverman NPC entrapped in its head. It has 4,000 / 5,000 health and is summoned with a Jellyfish Resonator while the player is near the Ocean during the day, but can be fought anywhere nearby afterwards. It summons various types of Royal Jellies to attack the player while launching bubbles that send the player high into the air, and will occasionally create a stationary cyclone to suck the player in. It will also outstretch tentacles to attack the player in Expert Mode when it loses enough life. Defeating Queen Jellyfish causes the Diverman NPC to drop right out of its head, and enables him to move in to an available house.
Queen Jellyfish drops a variety of jelly-themed weapons, as well as Pink Gel which is otherwise an item dropped from the rare Slime, Pinky. It also drops the Sea Breeze Pendant in Expert Mode, an accessory that grants several improvements to the player when they are underwater.
- Summoned by using 5
Music: Bat Ballad
The Viscount is a large royal bat with 6,000 / 8,000 health. It is summoned by using 5 Unholy Shards at a Blood Altar in the Cavern layer. The Viscount flies above the player, launching ricocheting echo pulses. It will also release multiple splashes of blood, stomp the ground stunning players, releases a ceiling breaking scream, summons life stealing Bitey Babies, and turns players into helpless bats in Expert Mode.
- Summoned with an
or breaking an
Music: Eye of the Storm
The Granite Energy Storm is a large granite core surrounded by energy. It has 7,000 / 9,000 health and has to be summoned by killing a Unstable Energy Anomaly in the Granite Caves or by using the Unstable Core in the Cavern layer. It uses a slightly delayed charge attack on the player while generating a circle of Coalesced Energy around itself, absorbing many of the player's attacks; this energy will then break off from the circle and home in on the player once seemingly destroyed. It will refresh its energy eventually if it is lost, and will also begin to launch energy balls from its core as it loses life, which deal high damage.
The Granite Energy Storm drops energized granite-themed weapons, as well as the Eye of the Storm in Expert Mode, an accessory that releases damaging energy in combat, which homes in on enemies.
- Summoned with an
or breaking a
Music: A Hero No More
The Buried Champion is a boss which resembles a winged Greek warrior. It has 7,000 / 9,000 health and has to be summoned by killing a Bizarre Rock Formation in the Marble Caves or by using the Ancient Blade in the Cavern layer. It will start out by using its blade to charge at the player, launch wind slices, and summon daggers from above. As it loses health, it will switch to using its bow, firing regular and elemental arrows at the player; and for its final phase will use its staff to fire volatile magic orbs while also summoning invulnerable echoes of itself to assist it. Defeating the Buried Champion is required for the Spiritualist NPC to move in.
- Summoned with a
Music: Interplanetary Harbinger
The Star Scouter is a large alien saucer which attacks with a series of purple lasers and beams. It is summoned with a Star Caller which can only be used in the Space biome. It has 8,000 / 10,000 health and is the most difficult boss before the Wall of Flesh. It will start out by using a vertical laser beam attack that creates horizontal energy waves when contacting a tile. It will add new attacks to its pattern as it loses life, including singular bursts of energy, homing graviton surges, and triplet energy beams. In Expert Mode, it also gains an unstable energy orb attack and at 25% life will release three Scouter Cores which attack the player and must be defeated for the fight to continue.
- Summoned with a
Music: Lost in the Blizzard
The Borean Strider is a boss that spawns randomly in the Snow biome during a Blizzard, or through use of a Strider's Tear. It has 10,000 / 15,000 health split over 2 phases. It leaps at the player, creating shockwaves as it lands, and shoots icicles at the player with increasing frequency as its health depletes. It also releases waves of snow and spawns Borean Hoppers and Borean Mytes during its second phase. Its attacks also inflict various ice-themed debuffs upon players.
The Borean Strider drops frost-themed weapons, as well as the Ice Bound Strider Hide in Expert Mode, an accessory that provides bonuses to players using weapons that inflict the Freezing debuff. Upon its defeat, new enemies will begin to spawn in underground Spider Caves.
- Summoned with a
Music: A Sight to Behold
The Fallen Beholder is a cephalopod-like boss that is summoned by consuming the Void Lens. It has 17,500 health / 25,000 health. It attacks with eradication beams and charge attacks at first. After losing 20% of its health it will begin to unleash a large spread of dark magic that debuffs the player but deals no damage. Sometimes it will summon Beholders to assist it. After losing 65% of its health it will begin to summon slow moving void gates that shoot powerful laser beams directly downwards.
Upon reaching 40% of its maximum health in Expert Mode, it will enter its second phase. As well as ramming into the player more often, it will unleash eradication beams and void gates far more quickly, its void gate will additionally cause a tidal wave of lava to rupture from below, and many of its attacks inflict a damaging burn on the player.
The Fallen Beholder drops infernal, demonic-themed weapons and accessories, as well as the Beholder Staff in Expert Mode, a summon weapon that calls forth Mini Beholders to blast energy at enemies. Upon its defeat, Lodestone Chunks and Valadium Chunks will generate throughout the world; this can happen up to three times per ore with each successive defeat.
- Summoned with a
Music: Harvest Night
The Lich is a skeleton-like boss summoned with the Grim Harvest Sigil. It has 25,000 / 40,000 health (40,000 split over 2 phases in Expert Mode). It rapidly fires flaming lasers from its eyes, rains down giant fireballs and spawns a bolt that stagnates mid-air or on tile impact.
Upon reaching 25% of its maximum health in Expert Mode, it will enter its second phase. In this form it rushes at the player with high speed and damage, shoots projectiles from its scythe, spawns colossal burning spirit balls and spawns the Phylactery of a Thousand Souls, which will fortify its defense greatly until it is killed.
The Lich drops deathly, soul-themed weapons and accessories, as well as Cursed Cloth, a crafting material that is used to make Lich armor. It also drops the Beholder's Gaze in Expert Mode, an accessory that can grant temporary guaranteed critical strikes for throwing weapons.
- Summoned by killing 3
or with an
Music: Beneath the Depths
The Forgotten One is a crab-like boss fought in the Aquatic Depths with 46,000 / 81,000 health split over 2 / 3 phases. It spawns automatically after collecting 3 Abyssal Shadows dropped by Aquatic Hallucinations, but after its defeat, Abyssal Shadows can be used to manually summon the boss. The Forgotten One has multiple attacks that become more numerous and frequent as its health depletes. Once seemingly defeated, the boss enters its second phase in which it has all of its attacks from the first but with added frequency and damage, and a new dark wave attack.
In Expert Mode, Forgotten One has a third and final phase in which its attacks become imbued with dark magic and it moves much faster, attempting to ram the player with incredible speed.
Forgotten One drops aquatic, abyssal-themed weapons, as well as the Abyssal Shell in Expert Mode, an accessory that can grant invulnerability at the cost of movement, life regeneration and attack capability.
- Summoned with a
Music: World's End
The Primordials is the final boss of the Thorium mod, summoned with the Doom Sayer's Coin. It consists of three bosses: Aquaius, The Endless Tide; Omnicide, The Life Defiler; and Slag Fury, The First Flame; each having their own set of attacks and forms. In total, the encounter has 462,000 / 793,000 health split over 3 / 4 bosses and several phases.
They each take turns assaulting the player with their own variety of attacks, requiring the player to adapt to the Primordial that is currently out; the Primordials not currently active will be intangible.
If all of them are defeated in Expert Mode, the final phase of the fight will begin.
The Primordials, when fully defeated, drops the three Primordial Essences, crafting materials used to make the most powerful gear available in the mod. It can also drop three unique items in Expert Mode: The Omega Core, an accessory that grants incredible speed; the Bow of Light, a rare true weapon with an extremely powerful charge attack; and the Dormant Hammer, which is used to craft the ultimate weapon of the mod, Mjölnir.
Mini-bosses are special enemies that can only be fought once per world. They are not as powerful as proper bosses, but are still more dangerous than normal enemies.
- Spawns naturally during a
Patch Werk is a mini-boss found during a Blood Moon. It has 500 / 1,000 health. Patch Werk slowly moves towards the player to deal damage. In Expert Mode, it will also use a damaging shockwave attack that summons Famished Maggots.
When killed, the Confused Zombie will be able to move in when there is sufficient housing, and the Abomination will replace Patch Werk in future Blood Moons. It will also drop two tome items: The Good Book, a basic Healer spell to assist allied players; and the Grave Buster, a magic weapon that summons skeletal hands from out of the ground.
- Spawns naturally in the
Corpse Bloom is a mini-boss that can be found rarely in the Underground Jungle after Skeletron has been defeated. It has 1000 / 2,000 health. It spawns up to 3 Corpse Petals, after which it fires a barrage of fungi. Every 25% health lost, it summons a Corpse Weed to attack the player.
When killed, the Druid will be able to move in when there is sufficient housing. It will also drop the Bloom Guard, a Healer support staff that can increase the defense and life regeneration of players affected by it, and the Weed Eater, a sentry that summons a poison spitting corpse weed.
- Summoned by interacting with a
The Illusionist is a mini-boss that is summoned by interacting with a Peculiar Mirror, which can be found rarely in the Dungeon. It has 500 / 1,000 health split into 3 healthbars, for a total of 1,500 / 3,000 health. The Illusionist teleports around the player while firing Illusion Glasses at the player. It also summons two Illusionist's Mirrors to orbit around it and deflect attacks, and Illusionist Decoys to attack and distract the player.
When killed, it will drop two items: the Mage Hand, a tool that can pick up and move items, Town NPCs, and enemies; and the Scrying Glass, a summoner accessory that increases the player's maximum number of sentries and their range of view.
Although many bosses can be fought in any particular order, their varying statistics and abilities may make certain boss fights less challenging than others at a given point in progression. Below lists the recommended order the player fights each boss in for the best experience.
- 220.127.116.11: Renamed "The Ragnarök" to The Primordials.
- 18.104.22.168: Added Peculiar Mirror, interacted with to summon the Illusionist.
- 22.214.171.124: Added Illusionist.
- 126.96.36.199: Reworked Star Scouter.
- 188.8.131.52: Added Viscount.
- 184.108.40.206: Reworked The Grand Thunder Bird.
- 220.127.116.11: Added Buried Champion.
- 18.104.22.168: Added Corpse Bloom.
- 22.214.171.124: Added an Expert Mode-exclusive phase to The Primordials.
- 126.96.36.199: Added Granite Energy Storm.
- 188.8.131.52: Added Star Scouter.
- 184.108.40.206: Added The Primordials.
- 220.127.116.11: Added Lich.
- 18.104.22.168: Added Queen Jellyfish and Borean Strider.
- 22.214.171.124: Added Fallen Beholder.
- 126.96.36.199: Added Patch Werk.