(I’ve replayed this mod three times, but I have yet to play the most recent update. Still, I believe what I have to say is relevant.)
This was originally a Terraria review, but I realized that I haven’t played vanilla/unmoded in forever. Instead, I’ve been playing this.
Calamity is one of the largest and most popular mods for Terraria. It can easily stand on its own, as it has more than enough content to have an entire playthrough different from the original. This mod expands upon every single aspect of the vanilla game. Featuring new mechanics, bosses, biomes, weapons, NPCs a new class, and even harder difficulty settings. They are all of extremely high quality, having features I didn’t even think were possible in Terraria. That’s the reason why I didn’t bother going back to unmoded, Calamity always had more content that was closer to what I wanted, i.e., extreme difficulty.
I highly recommend it to anyone that wants to experience modded Terraria, and/or is seeking a tougher challenge than vanilla. It does lean more toward the combat portion, especially bosses, so I wouldn’t recommend it to someone, who is a big fan of the calmer parts of the game.

𝐒𝐭𝐨𝐫𝐲
It doesn’t add a storyline to the game.
It does however have a lot of lore, that is used to, in a way, tie the world together. Most of the vanilla lore is overwritten and replaced with its version, which has its own characters and world history. It is quite fascinating and worth paying attention to.

𝐌𝐞𝐜𝐡𝐚𝐧𝐢𝐜𝐬 𝐚𝐧𝐝 𝐠𝐚𝐦𝐞𝐩𝐥𝐚𝐲
While a lot of content has been added, the gameplay loop has stayed the same. Defeat a boss, find out what has changed in the overworld, start getting the best gear and building the needed arena, defeat the next boss, and so on. Rudimentary, but it has given me countless hours of entertainment.
When looking at the bosses added and their gameplay, it does become apparent that Calamity leans more into the bullet hell genre, requiring precise movement and fast reaction times to stay alive. While a lot of attention has been put into making sure that the bosses are still, balanced and a fun fight, for those who just look at the cool weapons normal mode still exists. For me, however, that was why I kept coming back. The thrill of beating an extremely hard boss was like nothing else.
Overall, there isn't that much to say, it's just Terraria but more.

𝐆𝐫𝐚𝐩𝐡𝐢𝐜𝐬/𝐀𝐫𝐭𝐬𝐭𝐲𝐥𝐞
Calamity has incredible sprite and pixel art. Every weapon, tool, and enemy is meticulously crafted and somehow never feels out of place. There is a consistent tone and art direction that persists throughout the mod. The only place where a discrepancy can be spotted is when you compare them to vanilla. This is especially true for the endgame.

𝐀𝐭𝐦𝐨𝐬𝐩𝐡𝐞𝐫𝐞/𝐈𝐦𝐦𝐞𝐫𝐬𝐢𝐨𝐧
The way I felt during my playthrough and how I interpret the atmosphere is as such. You are on a mission to defeat your mortal enemy – the Tyrant, and no foe or abomination will stop you. Why? Who knows, revenge, power, justice, they are all possibilities. I believe it's open to interpretation.

𝐒𝐨𝐮𝐧𝐝𝐭𝐫𝐚𝐜𝐤
Back when I first played Calamity, I thought the OST was the coolest thing ever. I was right. It’s very rare to see a mod, not related to music, have such an incredible soundtrack. DM DOKURO and the other composers who have worked on the OST have created an amazing piece of music that I continue to listen to outside the game, just as much as I do inside.
Every boss and biome has a unique track that fits perfectly into the theme of said boss or biome.
My favourite is “Roar of the Jungle Dragon”. (I like lyrics in OSTs)

𝐅𝐢𝐧𝐚𝐥 𝐓𝐡𝐨𝐮𝐠𝐡𝐭𝐬
Okay, this is sort of unrelated to the mod. Back in one of my playthroughs, I had a mod installed that while carrying a certain amount of a potion in your inventory, gave you its buff, without needing to drink it. Another mod had a potion that gave a buff/debuff called heart attack. So, my character was running around fighting gods, while going through a permanent heart attack.

The difficulty modes are way too extreme in this mod in my opinion. It's honestly obnoxious. The expectations of the player are so unrealistic even for people who are good at the game. So, I recommend playing classic or expert, not revengeance or death, especially if you're playing for the first time. To play revengeance or death you need to be using the right loot for every single progression point in the game which requires game knowledge you have not obtained, and may not even obtain after one play-through because this mod adds a shit ton of stuff. You may not even finish a classic or expert mode playthrough there is so much to do, ... so playing on extreme difficulty is stupid. People who say "oh revengeance isn't that bad because they add new mechanics and weapons to balance it out" are smoking crack and have played terraria for 500+ hours minimum, don't listen to them.

Again this is obviously all opinion but I will die on this hill because as much as I enjoy this mod, it's my understanding that recommending people to play a high difficulty for a difficult and long mod in an already difficult and long game is braindead. Especially with the crazy boss re-works they just released. It's annoying that the mod overall difficulty and balance is changing over time, but that is something that happens in base Terraria too. It's hard to say if people who have rated the game highly from a past playthrough will always stand by their rating because this will be a whole different experience in 2 years time, just like it was completely different 2 years ago.

It's kinda unfortunate that base Terraria doesn't have some of the quality of life changes that this mod introduces, like infinite boss summons or crafting hermes boots or a horseshoe for example. I feel compelled to play this mod for all the crazy new additions, but revengeance is not the way, especially playing solo.

I'm also a little conflicted because I kind of combine calamity and other quality of life mods in my mind and forget that this is a review on just calamity, not calamity + other mods, so it's hard to say whether I would rather play base Terraria no mods or calamity with no quality of life mods. Overall, this mod definitely becomes equal to my feeling of normal Terraria because there are still bells and whistles to playing the game just like there are in normal Terraria, and there is still enough annoyance to rate it 7/10 instead of 9/10. Enough bloat for you to feel unreasonably confused or questioning why some parts of the mod were not combined because they are so similar or why some parts weren't cut altogether.