What is the lowest modifier you can have on a skill check (while staying within official rules)?

Question

I was watching some Critical Role last night, and one of the players had a -3 modifier to Insight (his Wisdom score of 7 didn't help). And that made me wonder:

What would be the lowest modifier you could create while staying within the normal, official rules?

Answer 1

-28 to all ability checks

I will give this a try and assume for the time being I can bring a nearly unlimited number of friends(or foes). I find it surprisingly hard to really push the modifier down, but this is my shot at it.

Lets start with the easy part, roll unlucky and you get a score of 3 in Dexterity, giving you a -4 for any ability checks you make.

Spells: -10

Now some enemies will cast a number of spells at our target (we assume all possible saves fail):

• Raise Dead giving you -4 on ability checks

• Synaptic Static You have to substract 1d6 from every ability check you make during the duration

Magic items: -2

Now we grab ourselves some magic items to even further curse ourselves (quite literally):

• Stone of Ill Luck (cursed): -2 to ability checks

Enemies and "friends" abilities: -12

Have a College ot Lore Bard use their cutting words feature on you, giving you a penalty of one roll of their bardic inspiration dice. We assume the bard is at least level 15, so their die is a d12.

End result

With that, we reach a total of -28 to all ability checks.