Hades

Story-related and cannot be missed.

Tartarus is the first biome in the game and is made up of 14 Chambers that you must clear. These chambers are mainly Encounters, rooms in which you'll fight multiple waves of enemies and receive a reward at the end. Above each chamber door is an icon that tells you what reward you'll get. It can be any of the following: Darkness, Gemstones, Chthonic Keys, Charon's Obols (coins), Nectar, Max Health upgrade, Pom of Power, or a Boon from an Olympian God, each with their own special icons. There are a couple other special chambers, including Charon's shop and other character's rooms (Sisyphus, Eurydice, and Patroclus).

When you complete a chamber, you'll have a choice of 1-3 chambers to enter next. In addition to the chamber reward, there can also be small icons accompanying them. A skull means there will be harder Armored enemies; a skull with one red arrow means it's a mini-boss chamber, and a skull with 2 red arrows means it's the final boss for that biome.

Note: The game's save system is built specifically to prevent abusing saves to alter chamber rewards. There is no way (that I'm aware of) to change the pre-determined rewards using saves/quitting/etc. The only way to alter the rewards of a chamber is by using Fated Authority stones/tokens, which you'll get by unlocking the last section of Nyx's Mirror of Night (see Nyx's Mirror ).

To escape Tartarus, you'll need to clear all 14 of its chambers. Its final Chamber is the boss fight, and this is the only region whose bosses change each time. The first few attempts you do, you will always face Megaera, but after a while, that changes, and you can face any one of the three sisters: Megaera, Alecto, or Tisiphone. Which one appears is absolutely random, and each sister has their own dedicated chamber and a unique flavor. The overall, major facets however are similar across all 3 Furies sisters; they share many traits and their fights are very similar. Their moves are the same kind of moves with individual details being different.

See below for more details on all 3 different boss fights [SPOILER WARNING]: 


Megaera:

You'll fight her in a large, purple room with a few spike traps around the map. For the first half of the fight, Meg only has 2 attacks. Her main move is her Dash Attack, which each sister has a version of. Meg's is simple; she'll target you and dash towards you, which will damage and stun you if she hits you. Her 2nd attack, the Whip Whirl, will occur if you're close to her, where she spins her whip around her, dealing damage in a large area. 

She has two other attacks that unlock at 75% health and 50% health. One is a circular bullet storm, where she rapidly fires a ton of shots at you in a 360 degree circular pattern, and the second is her Firebomb attack, the execution of which is very similar to the other two. She'll throw a cluster of 4 firebombs at you three times, but the location of the bombs will be displayed in advance as four large reddish circles on the ground. They will always appear directly under you wherever you are at that moment. You'll have a couple seconds to dash out of the area before the bombs land. 

Megaera occasionally spawns a couple enemies to help her. The first spawn will trigger once she's reached 75% health, then again at 50%, and lastly again at 25%, each time summoning a higher number of reinforcements. 

Once you get the hang of her moves, she becomes quite easy to defeat.


Alecto:

Like Megaera, she also has the same Dash/Lunge attack and the Whip Whirl. She also has an attack called Build Rage that increases her Rage meter and also comes with the same projectiles as Megaera's bullet storm. Alecto also summons reinforcments during the fight, but far less than Megaera, as she will only ever have 1-2 extra mobs at any one time.

She has 2 unique attacks that set her apart from her sisters. Instead of Megaera's quad firebomb-like attack, Alecto summons large circles on the ground that will chase you and, if you're caught in it, will shoot a large lightning bolt at you. You just have to keep running around, kiting the circles until the attack ends. The second unique attack is that she shoots a Blade Rift at you that can bounce around the room and can follow you. 

Finally, the last unique thing about Alecto is that she has a Rage meter. Her Rage grows throughout the fight and as her health decreases. Once her Rage meter is filled, she goes invulnerable for a few moments, and then all of her attacks either increase in rate of attack (lightning circles) or size (Blade Rifts), and her Dash and Whip Whirl release volleys of projectiles from her Build Rage attack (like Meg's bullet storm).



Tisiphone:

The main difference between her and her sisters is that the chamber in which you fight her changes at every 1/3rd of her health, and becomes smaller each time, thus limiting the space you have. This is done because her version of the firebombs she throws at you has far more than 4, and is spread over a much larger area. 

Other than that, she has the same Whip Whirl attack as the previous 2. Her version of the Dash Attack is what I call the Boomerang Dash; she will target you, dash towards you to stun you, quickly return to her original place, and then dash forwards again, this time shooting magic bullets at you as well. 

This is definitely the simplest fight out of the 3 of them.