Urschleim

Urschleim is a boss that appears in the Passage, far from The Core, at roughly 1240–1280 tiles away. When it is nearby, the player can hear a loud rumbling sound and the screen begins to shake. Its location can be revealed with an Urschleim Scanner, and after it has been defeated it can be summoned again anywhere in the Passage by using a Primeval Slime Cell.

Urschleim is a huge mass that slowly moves clockwise through the biome, passing through obstacles without destroying them. It does not use normal attacks, but contact with its body is instantly fatal. The fight revolves around its three glowing bulbs on the front: when all three are destroyed, its head becomes vulnerable for a short time and can then be damaged directly. After that brief window ends, the head retracts, the bulbs respawn, and the cycle repeats until the main body is defeated. A single vulnerability phase is limited to 25% of Urschleim’s maximum health, so the bulbs must be destroyed at least four times to finish the fight.

A few details make the encounter unusual:

• Its outer shell cannot be damaged; only the glowing bulbs and the head take damage.
• Attacks from behind are generally ineffective, since short-range weapons cannot reach and many ranged attacks will dissipate or bounce off as if they struck a block. The Stormbringer is the exception noted for rear damage.
• If the player dies during the fight, Urschleim’s head returns to full health, but the bulbs do not.
• Because Urschleim follows a fixed movement pattern, ranged weapons are especially effective for breaking the bulbs safely.
• It helps to either clear a long route through the Passage or learn the biome’s natural path in advance, since being caught in front of the boss usually means instant death.

The name Urschleim is German and means “primordial slime” or “original slime.”