The Zephyr Squid is a giant strangely colored squid with 3,800 (4,300) health that is fought in the Ocean biome. It is intended to be fought after the Eater of Worlds or the Brain of Cthulhu. It is summoned by fishing with the Coral Bait while in the Ocean biome. It attacks the player with two different projectiles that have their own unique properties.

The Zephyr Squid drops water-themed weapons and Zephyr Scales, which craft its weapons. In Expert Mode, it also drops the Zephyr Squid's Tentacle, which increases critical strike chance by 10%.

Glacieron is a menacing ice dragon with 5,200 (6,300) health that is fought in the Snow biome which can be summoned with the Frozen Food while in the Snow biome. It dashes at the player, fires varying ice projectiles, and will summon heavy blizzard when entering phase 2.

Glacieron drops snow-themed weapons and Glacial Pelts, which craft its weapons. In Expert Mode, it also drops the Glacieron Talon, which increases armor penetration by 10.

Dread is a strange monstrous alien with 14,000 (17,500) health that is fought before or alongside the Mechanical Bosses. It is summoned with the Beacon of Fear during nighttime. The boss is split into two phases: the first functioning similarly to Plantera and the second shedding its hooks, becoming airborne, and starting to directly attack the player.

Dread drops fear-themed weapons which can also be crafted with Dread Flame and Dread Scales. In Expert Mode it also drops the Core of Fear, which causes all projectiles to inflict the Dread debuff.

Xenanis is an ethereal demon with 52,000 (63,000) health that is fought after Plantera. It is summoned with the Ethereal Lantern while at nighttime. It uses various attacks, such as firing lasers in random directions and summoning portals that fire at the player.

Xenanis drops ethereal-themed weapons and some Ethereal Flesh, which craft its weapons, armor, and wings. In Expert Mode, it also drops the Xenanis' Core, which increases damage and crit by 15% when below half health.

Ultrum is the guardian of nature with 220,000 (380,000) health that is fought alongside Ignodium. He is summoned by using the Ultranium Sigil on the surface. He gains attacks and changes his behavior as his health decreases, including redirecting bolts, spiraling blasts, and walls of projectiles.

Ultrum drops Ultranium-themed weapons and Ultrum Essence Shards, which craft his weapons and Guardian items when used in conjunction with Ignodium Essence Shards. In Expert Mode, he also drops the Ultrum Relic Shard, which is used to craft the powerful endgame item, Guardian's Insignia.

Ignodium is the guardian of Hell with 220,000 (380,000) health that is fought alongside Ultrum. He is summoned by using the Nether Beacon while in The Underworld. He attacks using giga and flame blasts while occasionally charging at the player. At 50% health, he gains new attacks, such as using molten globs and horizontal walls.

Ignodium drops molten-themed weapons and Ignodium Essence Shards, which craft his weapons and Guardian items when used together with Ultrum Essence Shards. In Expert Mode, he also drops the Ignodium Relic Shard, which is used to craft the powerful endgame item, Guardian's Insignia.

Absolute Dread is a giant, even more, monstrous alien, with 280,000 (400,000) health, and is fought after both Ultrum and Ignodium have been defeated. It is summoned with the Beacon of Fear during nighttime after Ultrum and Ignodium are both defeated. The boss is a rematch of Dread, but more powerful.

Absolute Dread drops fear-themed weapons, along with Nightmare Fuel and Dread Scales, which craft its weapons. In Expert Mode, it also drops the True Core of Fear, which causes all projectiles to inflict the Dread (debuff) debuff, as well as causing critical hits to create flame bolts and getting hit will make the player create flame bolts.

Erebus is an Eldritch Worm with 385,000 (450,000) health, who is the final boss of Ultranium. It is summoned either at the end of the Abyssal Armageddon or with the Eldritch Worm Food during the Abyssal Armageddon event. It dashes across the screen and fires a lot of projectiles at the player.

Erebus drops shadow-themed weapons and Erebus Scales, which to craft endgame items, including Erebus' drops when used in conjunction with Tenebris Alloy and Other-Worldy Essence. In Expert Mode, it also drops the Heart of Erebus, which increases various stats as the player loses health.

Mind Flayer is a giant flayer wraith with 100,000 (200,000) health that spawns once during the Abyssal Armageddon Event. It attacks the player by moving quickly towards them and using various projectiles.

The Mind Flayer drops eldritch tentacle-themed weapons and Mind Flayer Blood, which is used to craft the Mind Flayer's drops and some other weapons. In Expert Mode, it also drops the Mind of the Mindflayer, which increases damage, gives you a special damaging aura and other bonuses.