Subtracts 1d8 from ALL damage rolls? So if I got hit by this and cast Magic Missile, I could end up doing no damage?

I was looking at the Cthonic Tiefling and was really disappointed because most of their spells beside False Life was kinda bad. Then I learned Chill Touch is now a melee cantrip that dealt 1d10>1d8, which I shrugged at because now it competed with Fire Bolt and had a better damage type (shrug, could be better but also the theme of Cthonic is melee tanky). Then I learned False Life got an extra d4 of Temp HP gained and I was a little happier. Then I learned this spell got buffed when people save against and I was actually pretty happy for Cthonic.

Overall, this spell getting a good effect when someone inevitably saves against it (because the saving throw is bad) made this spell really good for certain builds. Bladelocks, Eldritch Knights, and Arcane Tricksters will quite enjoy this spell! (The 1/3rd casters because they will enjoy having a spell that will help them no matter what their INT mod is).

Quote from Clathorus >>

Subtracts 1d8 from ALL damage rolls? So if I got hit by this and cast Magic Missile, I could end up doing no damage?

" has Disadvantage on Strength-based D20 Tests for the duration. During that time, it also subtracts 1d8 from all ITS damage rolls."

So only roll that is from the Strenght-based D20 tests. So if you use magic missile with your Strenght : yes, if not... doesnt concerne magic missile or spell whatsoever

I'm sorry, but I'm not seeing it. The fact that it's a new sentence, the use of the word "also", and that it just says "damage rolls", really makes it sound like it's not exclusively on the damage rolls of STR-based attacks.

Quote from Clathorus >>

I'm sorry, but I'm not seeing it. The fact that it's a new sentence, the use of the word "also", and that it just says "damage rolls", really makes it sound like it's not exclusively on the damage rolls of STR-based attacks.

 Agreed; however, this is a case of RAI versus RAW. I conjecture the intention is for reduced damage on strength-based damage (as is implied by the name of the spell). You would think the folks at WotC would clean up their language since they are paid to write this stuff.

So this spell works through walls? On invisible creatures? Anyone in full cover? Some random creature within range?  Does the damage subtraction of 1d8 on a failed save include area effect spells? Why is this so poorly written??

they change the rule for spell so meeting those requirements are a default assumption

I think it's clearly written for the target to subtract a d8 from all its damage rolls. The inclusion of the word "all" even seems like it is intentionally there to clear up ambiguity about whether that component of the ability only applies to strength based rolls. It neither feels strange thematically, nor from a balance perspective that the spell makes all of your offensive actions less potent.