It is impossible to discuss Mortal Shell out of the context of Dark Souls. This game is clearly a love letter to the Soulsborne genre yet it still manages to leave it's own mark. The whole concept of needing to find a "shell" (body) is a clever twist on the From Software formula. Since each shell has various strengths and weaknesses you many want to change them out depending on the circumstances. This is a bit of a shorter and more budget conscience experience but offers quite a fun challenge while it lasts.
Starting right off with the comparisons I must say the story in Mortal Shell is just as obscure as it is in the Souls games. You start your journey in a misty lake where you are taught the basic controls. Then you find yourself in a thin fragile husk crawling out into a dreary world. It is technically possible to play the entire game in this sorry state if you really want to a whole new level of challenge. For most of us you'll want to hop into the shell you find shortly after you emerge into the swamp. Once you make your way to the main hub you'll have access to visions which will show you where to find both other shells as well as new weapons.
Each shell has a difference balance of health and stamina which correlate to very different battle strategies. Additionally, each shell has a unique set of perks which can be unlock by spending Tar and Glimpses. Tar can be gathered simply by defeating enemies with tougher foes being worth the most. Every time you fall in battle you'll drop all your tar and have to run back to retrieve it. See anytime you die or access a save point all enemies will respawn. Glimpses are a little more challenging to gather but you don't lose them on death.
One thing I love about these kinds of games is the sense of discovery. Of the four different areas in the game two of them are quite open with lots of hidden items to gather. I found this game to be really challenging at first but once I was able to max out a weapon it felt much more reasonable. Of the four main bosses I really only found one of them really gave me any trouble and even then I just had to change my shell to defeat him. I clocked in at about 20 hours of playtime which seems pretty reasonable since this is merely a $30 game.
You don't need to be familiar with Dark Souls in order to enjoy Mortal Shell but at least if you do you'll have some idea of what you are getting into. The contrast between the first half of the game where you're weak and have no idea where you're going and the second half where you are quite strong and haves the maps mostly memorized is quite fascinating. You'd think a game should get more challenging as you progress, and does to an extent, but honestly your personal abilities and your weapon strength progress slightly fast than the game's challenge level. I have to say the quite liked the bosses in this game which isn't something I normally could say about From Software titles. I feel that the hardening ability and the different shells make this game different enough that Souls fans will find it enjoyable as a little change of pace.