[Lolzytripd]

Instead of +3 strength, could it perhaps be changed to either 
A) +3 main scaling stat of the weapon
Or
B) +2 main stat of the weapon scaling, +1 substats of the weapon scaling

[Shujin]

In all honesty, Demonic isn't all that interesting either way.

It probably need a whole revamp and not just minimal stats that even a firejewel does better.

[Snake]

Demonic could easily interact with Fear in some way, like a mini-Fallcall. Or maybe with debuffs.

Demonic - Increases damage against enemies by 5% per debuff they are suffering from, BUT, take 5% more damage per debuff you are suffering from. (Max. 40%).

[Krath]

I think most curses in general sure do need to be revamped, not just Demonic. They're supposed to be double-edged enchantments with big benefits and big downsides on top of being random drops and not able to be applied yourself, other than Haunted/Doomed. But Demonic is worse than the -jewels, Fleeting is an inferior Fated, Rustic is now a crappier version of Reaper, and Parasitic is just bad. Doomed actually is nice, but it's pretty much the only one other than Bloodsoaked for grinding.

To stay on the Demonic topic though, I wouldn't mind it being something like a % damage increase under certain conditions, yeah. 5% per debuff sounds fair and interesting and also has Hexer curse synergy, which is good since they like cursed weapons.

[renowner]

Misplaced needs to get the same buff fated got.
Demonic needs to give +5 Strength or +10 or something. Or just a big power buff. Or maybe some sort of bonus damage effect, that also hurts the caster when they use it.
Even Rustic now isn't as great now that Reaper is +5 power and +5 critical.