Firstly, what an incredible experience, I'm a huge fan of the Risk of Rain games and this felt almost like another remake but this time in terraria (also made me realise how similar the endgames of both Terraria and RoR2 are, mysterious King of the Moon is the final boss, have to fight four pillars worth of artificial constructs created by that King, also there being another boss in each game who is some sort of cultist for that King, and so on, I keep finding more similarities and it's blowing my mind, so thanks for prompting that lmao), and I loved how you also kind of kept the Voidling fight from RoR2 with the True Brain boss. That being said, I have couple of notes.

The True Brain is way, way too easy. I've won a few runs and every single time the Moon Lord boss felt kind of challenging while the True Brain was a walk in the park. I think it's to do with how simple the arena is, you can easily walk back and forth to avoid almost all it's attacks, and by the time you get to that stage in the game, you can tank the odd hit here and there and barely notice it. Maybe the solution is a more complex arena or maybe it's to increase the intensity and length of the attack patterns, I don't really know.

There are also two red items I want to comment on in terms of balance. Thermite Powder being the first, and ridiculously overpowered. Once you have it, you can just hide from the enemies each stage and they will eventually die, rendering almost all the non-boss rooms for the rest of the game completely harmless. I don't really know what the fix is there, maybe to change burn effects so that they can't actually finish off enemies and you have to be the one to hit them one way or another.

The other item is "item potentiometer". As a red item, a 2% chance to make enemies drop items is very underwhelming. I got this item towards the end of a run and it literally never triggered, rendering it a total waste of space in my inventory. On the other hand, using the Hero's Mod to give myself 5 of them near the start of a run felt great, and didn't break the game at all (most of the time you are just getting amber beads and memory foam and the like). The best fixes I can see are either to buff the percentage to 5 or 10% or to make the item a green (or even possibly both, there are precious few ways to obtain items other than at the end of a room, unlike a game like the original RoR or RoRR where the equivalent 56 leaf clover has similarly low rates, but there are items all over every stage)

That being said so far I haven't really encountered any other items that felt actually unfair one way or the other, you've done a really good job of making things just powerful enough and just rare enough that you feel like you've earned it even when you do pick up Shotgun Assembly Kit, Sniper Kit, and Salvaged Minigun all in the same run.

Awesome experience, I look forward to learning the best item combos and seeing what the future holds for this mod!