Deerclops

There is no boss you have to beat first. A Deer Thing used anywhere in a Snow or Ice biome brings it in, and unlike most summons it does not care what time it is. That freedom is a trap of sorts, since nothing in the progression stops you from ringing the bell on day two. Most people want a world evil boss down first, whichever one the world gave them, Eater of Worlds or else Brain of Cthulhu, but a confident player can take it far earlier and walk away with a large chunk of pre-Hardmode gear.

It also arrives on its own. The window is midnight during a Blizzard on the surface of a Snow biome, with a player standing in it who has either pushed their health past the first stretch of upgrades or picked up a little armor. The game wants that player isolated: town NPCs anywhere in a wide box around them cancel the spawn, as does any other boss or hostile event running at the time, with Blood Moons the one exception. After you have killed it once the roll drops to one chance in four on qualifying nights, so a farm is slow but possible.

Get too far from it and it wraps itself in shadow, turns dark purple with a magenta eye, and stops taking damage entirely. It stays like that until you come back inside its radius. That single rule writes the whole encounter. You cannot kite it, you cannot poke at it from the far end of an arena, and every second you spend running is a second of your own damage thrown in the bin. Plan for a close, ugly brawl.

When you are near, it rears back and slams, and a line of ice spikes tears out along the floor toward you, growing taller as it travels. Every third one behaves differently: slower to trigger, shorter reach, and it erupts to both sides at once. You can stand between its legs without taking contact damage, which is the cleanest way to sit out the first two waves, but staying there for the third is how that trick kills you. The freeze the spikes apply matters more than the hit, because it will drop you off a grappling hook in mid swing. Do not be crossing a gap when one lands.

Back away and the slam stops and the scooping starts. It tears up whatever it happens to be standing on and hurls the chunks in tall arcs that pass straight through your arena, so a ceiling does nothing about them. It also plants its feet and roars, hanging a long slow on everyone in a wide radius that you cannot dodge and cannot cleanse. The attack to respect is what happens if you hover above it for any length of time: it stops, roars, and puts a ring of shadow hands in the air around you. On Expert those hands are seeded throughout the fight rather than only overhead, some drifting in slowly, some curving in fast, some spinning in place, and they come thicker as its health drops.

A Warmth Potion works on it, which is unusual enough to be worth the slot, and a Hand Warmer removes the freeze outright. Because of the roar, movement gear such as an Anklet of the Wind or Magiluminescence with a Swiftness Potion does more for you than another damage accessory would. For the arena, lay a floor of platforms about two player heights off the ground with solid ramps at either end so it can climb up. Standing on that shelf it cannot use the debris throw at all, which deletes an entire attack from the fight. Last thing before you begin: move your spawn point out of the snow.

It does not vanish when it kills you. It wanders the biome it spawned in and picks the fight straight back up when you return, which is why the spawn point matters and why a Mediumcore death near it can spiral badly. Leaving the world gets rid of it, and so does letting a full in game day pass with nobody nearby. Nothing about the world changes when it dies. It is not a progression gate and it is not required for the boss slaying achievement, so the reward is the crossover haul and the coins, and you can bring it back as many times as you like with another summon item.