The Squirpent has awoken!
I always thought it was a really interesting mechanic that bosses in Terraria can spawn somewhat randomly. If you're playing for the first time, without help from the wiki or other players, you'll likely run into a lot of random encounters. Getting to 200 health and being ambushed by Eye of Cthulhu, standing on grass in the outer thirds of the world and encountering King Slime, killing all of the cultists by accident, etc. In my first exploration of a Terraria-style boss, I created the Squirpent, an Eater of Worlds v. Minecraft Squid hybrid, which spawns when you've killed your fair share of squids in the world.
Interestingly, an Eater of Worlds style boss is one of the bosses that translates almost directly into 3D. The way the boss moves remains relatively consistent with its 2D counterpart while still remaining threatening, because its ability to split really adds the extra layer of danger. On its own, it isn't too hard to dodge, but when it splits into several different segments it becomes a force to be reckoned with. It also works really nicely in a multiplayer setting, as everyone needs to be careful as to not split the boss.
The real challenge here is that the sound that worms make in Terraria actually becomes a pretty important tool in this fight. I used similar timings as Terraria (speeds up the frequency of sounds as the Squirpent gets closer, and slowing down the frequency when it is further away), and played the sound at specific locations around the player so that the directional audio can help locate where the Squirpent or its segments are in relation to the player (especially for headphone users).
Another interesting design choice made for this boss was to make the fight underwater. Minecraft has a nice swimming mechanic, so an underwater battle with tridents to toss and propel yourself through the water makes the fight very interesting. It's almost like flying in some sense, but much slower and more precise, and with the risk of drowning. This makes the Squirpent one of the bosses that I believe best represents Terraria's fighting style; arena creation is almost mandatory in this underwater battle, and the fight encourages creativity in how to deal with underwater mobility and breathing.
Additionally, since it can be quite dark underwater, I ended up opting for a glowing design to the boss which we implemented alongside magic weapons:
This gave a lot more visibility underwater, and really set the mood even more for the fight.